build.cmd uses either MSBuild 12 or MSBuild 14 and works on x64 & x86

Previously our repositories would build in either a Dev12 or a Dev14
Developer Command Prompt, but only if Dev12 (for MSBuild 12) was
on the box due to a hard-coded path to MSBuild 12.  This change
will use MSBuild 14 if MSBuild 12 is not available.  Also previously
this would look for MSBuild in the WOW Program Files path, this change
causes us to also look in the normal Program Files path for x86
Windows.

build.cmd

@@ -1,22 +1,30 @@
 @echo off
+setlocal
 
 :: Note: We've disabled node reuse because it causes file locking issues.
 ::       The issue is that we extend the build with our own targets which
 ::       means that that rebuilding cannot successully delete the task
-::       assembly.
+::       assembly. 
+
+:: Check prerequisites
+set _msbuildexe="%ProgramFiles(x86)%\MSBuild\12.0\Bin\MSBuild.exe"
+if not exist %_msbuildexe% set _msbuildexe="%ProgramFiles%\MSBuild\12.0\Bin\MSBuild.exe"
+if not exist %_msbuildexe% set _msbuildexe="%ProgramFiles(x86)%\MSBuild\14.0\Bin\MSBuild.exe"
+if not exist %_msbuildexe% set _msbuildexe="%ProgramFiles%\MSBuild\14.0\Bin\MSBuild.exe"
+if not exist %_msbuildexe% echo Error: Could not find MSBuild.exe.  Please see https://github.com/dotnet/corefx/blob/master/docs/Developers.md for build instructions. && goto :eof
 
 :: Log build command line
-set buildprefix=echo
-set buildpostfix=^> "%~dp0msbuild.log"
+set _buildprefix=echo
+set _buildpostfix=^> "%~dp0msbuild.log"
 call :build %*
 
 :: Build
-set buildprefix=
-set buildpostfix=
+set _buildprefix=
+set _buildpostfix=
 call :build %*
 
 goto :eof
 
 :build
-%buildprefix% "%ProgramFiles(x86)%\MSBuild\12.0\Bin\MSBuild.exe" "%~dp0build.proj" /nologo /maxcpucount /verbosity:minimal /nodeReuse:false /fileloggerparameters:Verbosity=diag;LogFile="%~dp0msbuild.log";Append %* %buildpostfix%
-goto :eof
+%_buildprefix% %_msbuildexe% "%~dp0build.proj" /nologo /maxcpucount /verbosity:minimal /nodeReuse:false /fileloggerparameters:Verbosity=diag;LogFile="%~dp0msbuild.log";Append %* %_buildpostfix%
+goto :eof