Add VS 2010 and MinGW to AppVeyor build matrix

VS 2010 and MinGW are the two trickiest Windows environments so this
should hopefully catch more mistakes.  This is probably not the right
way to set up any of this, but it works for now.

This is also a first step towards building our release binaries with CI.

[skip travis]

.appveyor.yml

@@ -1,3 +1,6 @@
+image:
+    - Visual Studio 2015
+    - Visual Studio 2019
 branches:
     only:
         - ci
@@ -5,17 +8,50 @@ branches:
         - 3.3-stable
 skip_tags: true
 environment:
-    CFLAGS: /WX
     matrix:
-        - BUILD_SHARED_LIBS: ON
-        - BUILD_SHARED_LIBS: OFF
+        - GENERATOR: MinGW Makefiles
+          BUILD_SHARED_LIBS: ON
+          CFLAGS: -Werror
+        - GENERATOR: MinGW Makefiles
+          BUILD_SHARED_LIBS: OFF
+          CFLAGS: -Werror
+        - GENERATOR: Visual Studio 10 2010
+          BUILD_SHARED_LIBS: ON
+          CFLAGS: /WX
+        - GENERATOR: Visual Studio 10 2010
+          BUILD_SHARED_LIBS: OFF
+          CFLAGS: /WX
+        - GENERATOR: Visual Studio 16 2019
+          BUILD_SHARED_LIBS: ON
+          CFLAGS: /WX
+        - GENERATOR: Visual Studio 16 2019
+          BUILD_SHARED_LIBS: OFF
+          CFLAGS: /WX
 matrix:
     fast_finish: true
-build_script:
-    - mkdir build
-    - cd build
-    - cmake -DCMAKE_VERBOSE_MAKEFILE=ON -DBUILD_SHARED_LIBS=%BUILD_SHARED_LIBS% ..
-    - cmake --build .
+    exclude:
+        - image: Visual Studio 2015
+          GENERATOR: Visual Studio 16 2019
+        - image: Visual Studio 2019
+          GENERATOR: Visual Studio 10 2010
+        - image: Visual Studio 2019
+          GENERATOR: MinGW Makefiles
+for:
+-
+    matrix:
+        except:
+            - GENERATOR: Visual Studio 10 2010
+    build_script:
+        - set PATH=%PATH:C:\Program Files\Git\usr\bin=C:\MinGW\bin%
+        - cmake -S . -B build -G "%GENERATOR%" -DBUILD_SHARED_LIBS=%BUILD_SHARED_LIBS%
+        - cmake --build build
+-
+    matrix:
+        only:
+            - GENERATOR: Visual Studio 10 2010
+    build_script:
+        - cmake -S . -B build -G "%GENERATOR%" -DBUILD_SHARED_LIBS=%BUILD_SHARED_LIBS%
+        - cmake --build build --target glfw
 notifications:
     - provider: Email
       to: