[msbuild] Fix metal targets for Xcode8

[VincentDondain]

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).

[rolfbjarne]

Did you test this on Xcode 8.0?

[spouliot]

If possible this should be backported (or re-written) to xcode8.3.

[VincentDondain]

@rolfbjarne yes tried with Xcode8+ (8.0, 8.1... 8.3 beta 3) and Xcode 7.3.1 for backward compatibility.

That command change (skip the intermediary target) happened in Xcode 8.0 we just didn't update our msbuild targets so the main goal here was to have both Xcode 7 and Xcode 8+ working.

[VincentDondain]

@spouliot yes I'll back port to xcode8.3 when it's merged 👍

[jstedfast]

Logic looks excellent but I'm thinking that having the variable be _IsXcode7 might be better since _IsXcode8 suggests an exact version rather than >= 8.0.

Other than that, I love it!

[VincentDondain]

@jstedfast well similarly IsXcode7 suggests an exact version rather than >= 7.0 (;

Also IsXcode8 suggests it's Xcode8.X not Xcode8.0.

I can rename it to IsXcode8Plus if you want, or IsXcode8AndAbove but I think it's fine right now.

[monojenkins]

Build success

[jstedfast]

Well, I just mean that IsXcode7 (or IsXcode7x if you prefer) is better long-term in the sense that presumably Xcode9 will have Xcode8 behavior but IsXcode8 or IsXcode8x is confusing when it matches Xcode 9.x

[VincentDondain]

IsXcode7 or IsXcode7Plus = Xcode 7.0, Xcode 7.3.1, Xcode 8.0, Xcode 9.0, etc etc...

The command change happened in Xcode8.0 so we can logically name the bool either Xcode8.0+ (which includes Xcode 9.0, Xcode 10.0 etc) or Xcode8.0- (non inclusive) so it's everything before Xcode8.0.

So naming the variable IsXcode7 first of all doesn't cover Xcode6 for instance and then if we don't want to have it sound like it's an exact version requirement of Xcode7.0, and name it IsXcode7AndAbove, we're also wrong because it includes Xcode 8 and 9 (:

#namingishard

I think IsXcode8AndAbove is what we want here (:

[jstedfast]

We can look at this again when Xcode9 comes out I guess and see. At that point, we could probably drop support for Xcode7 as well.