[CMake] Move the setting of LLVM_COMPILER_IS_GCC_COMPATIBLE to a separate file

[john.brawn]

Summary

Currently LLVM_COMPILER_IS_GCC_COMPATIBLE is set as a side-effect of determining the stdlib to use in HandleLLVMStdlib, which causes problems when attempting to use AddLLVM from an installed LLVM toolchain, as HandleLLVMStdlib is not used. Move the setting of this variable into DetermineGCCCompatible and include that from both AddLLVM and HandleLLVMStdlib.

[chapuni]

Looks good.

That said, I think rather we may reorganize handling individual options.
I don't have strong opinions for this.

llvm/trunk/cmake/modules/AddLLVM.cmake

 include(LLVMProcessSources)
 include(LLVM-Config)
+include(DetermineGCCCompatible)
 
 function(llvm_update_compile_flags name)
   get_property(sources TARGET ${name} PROPERTY SOURCES)
   if("${sources}" MATCHES "\\.c(;|$)")
     set(update_src_props ON)
   endif()
 
   # LLVM_REQUIRES_EH is an internal flag that individual

llvm/trunk/cmake/modules/DetermineGCCCompatible.cmake

+# Determine if the compiler has GCC-compatible command-line syntax.
+
+if(NOT DEFINED LLVM_COMPILER_IS_GCC_COMPATIBLE)
+  if(CMAKE_COMPILER_IS_GNUCXX)
+    set(LLVM_COMPILER_IS_GCC_COMPATIBLE ON)
+  elseif( MSVC )
+    set(LLVM_COMPILER_IS_GCC_COMPATIBLE OFF)
+  elseif( "${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang" )
+    set(LLVM_COMPILER_IS_GCC_COMPATIBLE ON)
+  endif()
+endif()

llvm/trunk/cmake/modules/HandleLLVMStdlib.cmake

 # This CMake module is responsible for setting the standard library to libc++
 # if the user has requested it.
 
+include(DetermineGCCCompatible)
+
 if(NOT DEFINED LLVM_STDLIB_HANDLED)
   set(LLVM_STDLIB_HANDLED ON)
 
-  if(CMAKE_COMPILER_IS_GNUCXX)
-    set(LLVM_COMPILER_IS_GCC_COMPATIBLE ON)
-  elseif( MSVC )
-    set(LLVM_COMPILER_IS_GCC_COMPATIBLE OFF)
-  elseif( "${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang" )
-    set(LLVM_COMPILER_IS_GCC_COMPATIBLE ON)
-  endif()
-
   function(append value)
     foreach(variable ${ARGN})
       set(${variable} "${${variable}} ${value}" PARENT_SCOPE)
     endforeach(variable)
   endfunction()
 
   include(CheckCXXCompilerFlag)
   if(LLVM_ENABLE_LIBCXX)