Don't pass -fms-compatibility-version flag during build

[ismail]

Summary

Since some time clang itself figures out the default for ms-compatibility-version and uses it. Trying to figure it out during build is redundant and also will not work when the environment variable VSINSTALLDIR is not defined (which is not defined if you don't install whole Visual Studio but use Visual C++ Build Tools package).

Tested by bootstrapping clang with clang-cl.

[hans]

lgtm if it works :-)

llvm/trunk/cmake/modules/HandleLLVMOptions.cmake

if( MSVC )
 
   # "Generate Intrinsic Functions".
   append("/Oi" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 
   # "Enforce type conversion rules".
   append("/Zc:rvalueCast" CMAKE_CXX_FLAGS)
 
   if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
-    # Find and run MSVC (not clang-cl) and get its version. This will tell
-    # clang-cl what version of MSVC to pretend to be so that the STL works.
-    execute_process(COMMAND "$ENV{VSINSTALLDIR}/VC/bin/cl.exe"
-      OUTPUT_QUIET
-      ERROR_VARIABLE MSVC_COMPAT_VERSION
-      )
-    string(REGEX REPLACE "^.*Compiler Version ([0-9.]+) for .*$" "\\1"
-      MSVC_COMPAT_VERSION "${MSVC_COMPAT_VERSION}")
-    append("-fms-compatibility-version=${MSVC_COMPAT_VERSION}"
-      CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
-  endif()
-
-  if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
     # clang-cl and cl by default produce non-deterministic binaries because
     # link.exe /incremental requires a timestamp in the .obj file.  clang-cl
     # has the flag /Brepro to force deterministic binaries. We want to pass that
     # whenever you're building with clang unless you're passing /incremental.
     # This checks CMAKE_CXX_COMPILER_ID in addition to check_cxx_compiler_flag()
     # because cl.exe does not emit an error on flags it doesn't understand,
     # letting check_cxx_compiler_flag() claim it understands all flags.
     check_cxx_compiler_flag("/Brepro" SUPPORTS_BREPRO)