Improve BUILDING.md regarding which Windows toolchains work

[briansmith]

• Improve the documentation on building to explain that right now -msvc is what is known to work and that that requires specifically the -msvc Rust toolchain.
• Include a link to the issue for getting the -gnu toolchain into working order.
• Include a link about the rewrite of the build.rs that is happening in [WIP] Port build system to use only build.rs on unix #321.

[Philipp91]

Another note:

On my laptop, I figured I wouldn't need the whole Visual Studio, and as rustup suggests (https://github.com/rust-lang-nursery/rustup.rs/#other-installation-methods), I installed only the Visual C++ Build Tools 2015, which took long enough and 3GB ...
The ring build failed because it couldn't find Microsoft.Cpp.Default.props, which is in C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\v140\Microsoft.Cpp.Default.props on my installation. It turned out that some registry keys were missing, which I copied over from my desktop PC:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSBuild\ToolsVersions\14.0]
"VCTargetsPath"="$([MSBuild]::ValueOrDefault('$(VCTargetsPath)','$(MSBuildExtensionsPath32)\\Microsoft.Cpp\\v4.0\\V140\\'))"
"VCTargetsPath10"="$([MSBuild]::ValueOrDefault('$(VCTargetsPath10)','$(MSBuildExtensionsPath32)\\Microsoft.Cpp\\v4.0\\'))"
"VCTargetsPath11"="$([MSBuild]::ValueOrDefault('$(VCTargetsPath11)','$(MSBuildExtensionsPath32)\\Microsoft.Cpp\\v4.0\\V110\\'))"
"VCTargetsPath12"="$([MSBuild]::ValueOrDefault('$(VCTargetsPath12)','$(MSBuildExtensionsPath32)\\Microsoft.Cpp\\v4.0\\V120\\'))"
"VCTargetsPath14"="$([MSBuild]::ValueOrDefault('$(VCTargetsPath14)','$(MSBuildExtensionsPath32)\\Microsoft.Cpp\\v4.0\\V140\\'))"

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSBuild\ToolsVersions\14.0\10.0]
"VCTargetsPath"="$([MSBuild]::ValueOrDefault('$(VCTargetsPath)','$(MSBuildExtensionsPath32)\\Microsoft.Cpp\\v4.0\\'))"

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSBuild\ToolsVersions\14.0\11.0]
"VCTargetsPath"="$([MSBuild]::ValueOrDefault('$(VCTargetsPath)','$(MSBuildExtensionsPath32)\\Microsoft.Cpp\\v4.0\\V110\\'))"
"VCTargetsPath10"="$([MSBuild]::ValueOrDefault('$(VCTargetsPath10)','$(MSBuildExtensionsPath32)\\Microsoft.Cpp\\v4.0\\'))"
"VCTargetsPath11"="$([MSBuild]::ValueOrDefault('$(VCTargetsPath11)','$(MSBuildExtensionsPath32)\\Microsoft.Cpp\\v4.0\\V110\\'))"

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSBuild\ToolsVersions\14.0\12.0]
"VCTargetsPath"="$([MSBuild]::ValueOrDefault('$(VCTargetsPath)','$(MSBuildExtensionsPath32)\\Microsoft.Cpp\\v4.0\\V120\\'))"
"VCTargetsPath10"="$([MSBuild]::ValueOrDefault('$(VCTargetsPath10)','$(MSBuildExtensionsPath32)\\Microsoft.Cpp\\v4.0\\'))"
"VCTargetsPath11"="$([MSBuild]::ValueOrDefault('$(VCTargetsPath11)','$(MSBuildExtensionsPath32)\\Microsoft.Cpp\\v4.0\\V110\\'))"
"VCTargetsPath12"="$([MSBuild]::ValueOrDefault('$(VCTargetsPath12)','$(MSBuildExtensionsPath32)\\Microsoft.Cpp\\v4.0\\V120\\'))"

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSBuild\ToolsVersions\14.0\14.0]
"VCTargetsPath"="$([MSBuild]::ValueOrDefault('$(VCTargetsPath)','$(MSBuildExtensionsPath32)\\Microsoft.Cpp\\v4.0\\V140\\'))"
"VCTargetsPath10"="$([MSBuild]::ValueOrDefault('$(VCTargetsPath10)','$(MSBuildExtensionsPath32)\\Microsoft.Cpp\\v4.0\\'))"
"VCTargetsPath11"="$([MSBuild]::ValueOrDefault('$(VCTargetsPath11)','$(MSBuildExtensionsPath32)\\Microsoft.Cpp\\v4.0\\V110\\'))"
"VCTargetsPath12"="$([MSBuild]::ValueOrDefault('$(VCTargetsPath12)','$(MSBuildExtensionsPath32)\\Microsoft.Cpp\\v4.0\\V120\\'))"
"VCTargetsPath14"="$([MSBuild]::ValueOrDefault('$(VCTargetsPath14)','$(MSBuildExtensionsPath32)\\Microsoft.Cpp\\v4.0\\V140\\'))"

While this is ultimately a MSBuild bug (probably), one might come accross this when installing rustup and MSBuild for ring (or other Rust projects that use the msvc toolchain). ring can't fix this, but maybe the rustup documentation (which references the Visual C++ Build Tools 2015) could hint at this (not sure where exactly to report that) and ring could then point at them.

[briansmith]

@Philipp91 Thanks! I've filed #337 for making Visual C++ Build Tools 2015 work.

Regarding this issue, I updated the documentation in e45ee6e.

Thanks again for your help!