For windows, ideSdkPath was defined twice.
Also the base tasks are being built only on the Mac, VSW get those from the bundle.zip, so the Windows check should be done at runtime.
First: only run `EnsureSdkPath` if `EnsureAppleSdkRoot` passed.
Both are based on similar install checks, `SdkIsInstalled` (used for `SdkIsInstalled`) is empty
if `IsInstalled` (used by `EnsureAppleSdkRoot`) is false.
This avoid having 2 error messages when only 1 at a time is needed.
Second: improved `EnsureAppleSdkRoot` error message mentioning the wrong Xcode developer path (which triggered the error)
as well as the exact way to fix it.
Fixes bug #52982: [iOS]Metal samples fail to build with Xcode8.3
(https://bugzilla.xamarin.com/show_bug.cgi?id=52982)
Basically with Xcode8 Apple stopped using an intermediary step to generate the
default.metallib. This was what our `_ForgeMetal` target was doing, generate a `default.metal-ar`
file which was used as input for `_TemperMetal` and then generate the default.metallib.
Instead with Xcode8 you can just give Shaders.air directly to the metallib tool.
The fix in this commit is made in such a way that it still supports Xcode7.
if !Xcode8 then don't change anything.
if Xcode8+ then have `_ForgedMetal` output equal `@(_SmeltedMetal)` (basically skip the _ForgeMetal target).
Vincent Dondain
Rolf Bjarne Kvinge