CMake: default Visual Studio generator has changed

Correct some wording and inform users regarding the Visual Studio
changes (from V16.6) to the default generator.

Subsequent commits ensure that Git for Windows can be directly
opened in modern Visual Studio without needing special configuration
of the CMakeLists settings.

It appeares that internally Visual Studio creates it's own version of the
.sln file (etc.) for extension tools that expect them.

The large number of references below document the shifting of Visual Studio
default and CMake setting options.

refs: https://docs.microsoft.com/en-us/search/?scope=C%2B%2B&view=msvc-150&terms=Ninja

1. https://docs.microsoft.com/en-us/cpp/linux/cmake-linux-configure?view=msvc-160
(note the linux bit)
 "In Visual Studio 2019 version 16.6 or later ***, Ninja is the default
generator for configurations targeting a remote system or WSL. For more
information, see this post on the C++ Team Blog
[https://devblogs.microsoft.com/cppblog/linux-development-with-visual-studio-first-class-support-for-gdbserver-improved-build-times-with-ninja-and-updates-to-the-connection-manager/].

For more information about these settings, see CMakeSettings.json reference
[https://docs.microsoft.com/en-us/cpp/build/cmakesettings-reference?view=msvc-160]."

2. https://docs.microsoft.com/en-us/cpp/build/cmake-presets-vs?view=msvc-160
"CMake supports two files that allow users to specify common configure,
build, and test options and share them with others: CMakePresets.json
and CMakeUserPresets.json."

" Both files are supported in Visual Studio 2019 version 16.10 or later.
***"
3. https://devblogs.microsoft.com/cppblog/linux-development-with-visual-studio-first-class-support-for-gdbserver-improved-build-times-with-ninja-and-updates-to-the-connection-manager/
" Ninja has been the default generator (underlying build system) for
CMake configurations targeting Windows for some time***, but in Visual
Studio 2019 version 16.6 Preview 3*** we added support for Ninja on Linux."

4. https://docs.microsoft.com/en-us/cpp/build/cmakesettings-reference?view=msvc-160
" `generator`: specifies CMake generator to use for this configuration.
May be one of:

    Visual Studio 2019 only:
        Visual Studio 16 2019
        Visual Studio 16 2019 Win64
        Visual Studio 16 2019 ARM

    Visual Studio 2017 and later:
        Visual Studio 15 2017
        Visual Studio 15 2017 Win64
        Visual Studio 15 2017 ARM
        Visual Studio 14 2015
        Visual Studio 14 2015 Win64
        Visual Studio 14 2015 ARM
        Unix Makefiles
        Ninja

Because Ninja is designed for fast build speeds instead of flexibility
and function, it is set as the default. However, some CMake projects may
be unable to correctly build using Ninja. If this occurs, you can
instruct CMake to generate Visual Studio projects instead.

To specify a Visual Studio generator in Visual Studio 2017, open the
settings editor from the main menu by choosing CMake | Change CMake
Settings. Delete "Ninja" and type "V". This activates IntelliSense,
which enables you to choose the generator you want."

"To specify a Visual Studio generator in Visual Studio 2019, right-click
on the CMakeLists.txt file in Solution Explorer and choose CMake
Settings for project > Show Advanced Settings > CMake Generator.

When the active configuration specifies a Visual Studio generator, by
default MSBuild.exe is invoked with` -m -v:minimal` arguments."

5. https://docs.microsoft.com/en-us/cpp/build/cmake-presets-vs?view=msvc-160#enable-cmakepresetsjson-integration-in-visual-studio-2019
"Enable CMakePresets.json integration in Visual Studio 2019

CMakePresets.json integration isn't enabled by default in Visual Studio
2019. You can enable it for all CMake projects in Tools > Options >
CMake > General: (tick a box)" ... see more.

6. https://docs.microsoft.com/en-us/cpp/build/cmakesettings-reference?view=msvc-140
(whichever v140 is..)
"CMake projects are supported in Visual Studio 2017 and later."

7. https://docs.microsoft.com/en-us/cpp/overview/what-s-new-for-cpp-2017?view=msvc-150
"Support added for the CMake Ninja generator."

8. https://docs.microsoft.com/en-us/cpp/overview/what-s-new-for-cpp-2017?view=msvc-150#cmake-support-via-open-folder
"CMake support via Open Folder
Visual Studio 2017 introduces support for using CMake projects without
converting to MSBuild project files (.vcxproj). For more information,
see CMake projects in Visual
Studio[https://docs.microsoft.com/en-us/cpp/build/cmake-projects-in-visual-studio?view=msvc-150].
Opening CMake projects with Open Folder automatically configures the
environment for C++ editing, building, and debugging." ... +more!

9. https://docs.microsoft.com/en-us/cpp/build/cmake-presets-vs?view=msvc-160#supported-cmake-and-cmakepresetsjson-versions
"Visual Studio reads and evaluates CMakePresets.json and
CMakeUserPresets.json itself and doesn't invoke CMake directly with the
--preset option. So, CMake version 3.20 or later isn't strictly required
when you're building with CMakePresets.json inside Visual Studio. We
recommend using CMake version 3.14 or later."

10. https://docs.microsoft.com/en-us/cpp/build/cmake-presets-vs?view=msvc-160#enable-cmakepresetsjson-integration-in-visual-studio-2019
"If you don't want to enable CMakePresets.json integration for all CMake
projects, you can enable CMakePresets.json integration for a single
CMake project by adding a CMakePresets.json file to the root of the open
folder. You must close and reopen the folder in Visual Studio to
activate the integration.

11. https://docs.microsoft.com/en-us/cpp/build/cmake-presets-vs?view=msvc-160#default-configure-presets
***(doesn't actually say which version..)
"Default Configure Presets
If no CMakePresets.json or CMakeUserPresets.json file exists, or if
CMakePresets.json or CMakeUserPresets.json is invalid, Visual Studio
will fall back*** on the following default Configure Presets:

Windows example
JSON
{
  "name": "windows-default",
  "displayName": "Windows x64 Debug",
  "description": "Sets Ninja generator, compilers, x64 architecture,
build and install directory, debug build type",
  "generator": "Ninja",
  "binaryDir": "${sourceDir}/out/build/${presetName}",
  "architecture": {
    "value": "x64",
    "strategy": "external"
  },
  "cacheVariables": {
    "CMAKE_BUILD_TYPE": "Debug",
    "CMAKE_INSTALL_PREFIX": "${sourceDir}/out/install/${presetName}"
  },
  "vendor": {
    "microsoft.com/VisualStudioSettings/CMake/1.0": {
      "hostOS": [ "Windows" ]
    }
  }
},
"

Signed-off-by: Philip Oakley <philipoakley@iee.email>

contrib/buildsystems/CMakeLists.txt

@@ -14,6 +14,11 @@ Note: Visual Studio also has the option of opening `CMakeLists.txt`
 directly; Using this option, Visual Studio will not find the source code,
 though, therefore the `File>Open>Folder...` option is preferred.
 
+Visual Studio does not produce a .sln solution file nor the .vcxproj files
+that may be required by VS extension tools.
+
+To generate the .sln/.vcxproj files run CMake manually, as described below.
+
 Instructions to run CMake manually:
 
     mkdir -p contrib/buildsystems/out
@@ -22,7 +27,7 @@ Instructions to run CMake manually:
 
 This will build the git binaries in contrib/buildsystems/out
 directory (our top-level .gitignore file knows to ignore contents of
-this directory).
+this directory). The project .sln and .vcxproj files are also generated.
 
 Possible build configurations(-DCMAKE_BUILD_TYPE) with corresponding
 compiler flags
@@ -35,17 +40,16 @@ empty(default) :
 NOTE: -DCMAKE_BUILD_TYPE is optional. For multi-config generators like Visual Studio
 this option is ignored
 
-This process generates a Makefile(Linux/*BSD/MacOS) , Visual Studio solution(Windows) by default.
+This process generates a Makefile(Linux/*BSD/MacOS), Visual Studio solution(Windows) by default.
 Run `make` to build Git on Linux/*BSD/MacOS.
 Open git.sln on Windows and build Git.
 
-NOTE: By default CMake uses Makefile as the build tool on Linux and Visual Studio in Windows,
-to use another tool say `ninja` add this to the command line when configuring.
-`-G Ninja`
-
 NOTE: By default CMake will install vcpkg locally to your source tree on configuration,
 to avoid this, add `-DNO_VCPKG=TRUE` to the command line when configuring.
 
+The Visual Studio default generator changed in v16.6 from its Visual Studio
+implemenation to `Ninja` This required changes to many CMake scripts.
+
 ]]
 cmake_minimum_required(VERSION 3.14)