Add the -prelease switch to vswhere in build scripts to allow them to find prerelease versions of Visual Studio. This fixes building on a machine that only has C++ installed in a preview build of VS 2017. (#4696)

buildscripts/buildvars-setup.cmd

@@ -118,7 +118,7 @@ if defined VisualStudioVersion goto :RunVCVars
 
 set _VSWHERE="%ProgramFiles(x86)%\Microsoft Visual Studio\Installer\vswhere.exe"
 if exist %_VSWHERE% (
-  for /f "usebackq tokens=*" %%i in (`%_VSWHERE% -latest -property installationPath`) do set _VSCOMNTOOLS=%%i\Common7\Tools
+  for /f "usebackq tokens=*" %%i in (`%_VSWHERE% -latest -prerelease -property installationPath`) do set _VSCOMNTOOLS=%%i\Common7\Tools
 )
 if not exist "%_VSCOMNTOOLS%" goto :MissingVersion
 

tests/runtest.cmd

@@ -107,7 +107,7 @@ if defined VisualStudioVersion goto :RunVCVars
 
 set _VSWHERE="%ProgramFiles(x86)%\Microsoft Visual Studio\Installer\vswhere.exe"
 if exist %_VSWHERE% (
-  for /f "usebackq tokens=*" %%i in (`%_VSWHERE% -latest -property installationPath`) do set _VSCOMNTOOLS=%%i\Common7\Tools
+  for /f "usebackq tokens=*" %%i in (`%_VSWHERE% -latest -prerelease -property installationPath`) do set _VSCOMNTOOLS=%%i\Common7\Tools
 )
 
 call "%_VSCOMNTOOLS%\VsDevCmd.bat"