[PATCH] [Clang] Implement basic MinGW32 version-independent installation detection

[Chris Kay]

>>! In D4177#9, @yaron.keren wrote:
> You can disable building the tools with -DLLVM_BUILD_TOOLS=OFF on the CMake command line. 
> More build options: http://llvm.org/docs/CMake.html
> 
> I am building LLVM with Visual C++ 2013 routinely and I think one of the build bots does use MinGW.
> Which compiler are you using? MinGW? which version?

I'm using MinGW64 with GCC 4.8.1-5.
I've just pulled from master again and it looks like today's CMake problem is an infinite loop:



  Target triple: i686-pc-mingw32
  Native target architecture is X86
  Threads enabled.
  Doxygen disabled.
  Sphinx disabled.
  Constructing LLVMBuild project information
  LLVMHello ignored -- Loadable modules not supported on this platform.
  Targeting X86
  BugpointPasses ignored -- Loadable modules not supported on this platform.
  CMake Warning at C:/Program Files (x86)/CMake 2.8/share/cmake-2.8/Modules/CPack.cmake:289 (message):
    CPack.cmake has already been included!!
  Call Stack (most recent call first):
    CMakeLists.txt:79 (include)
  
  
  Target triple: i686-pc-mingw32
  Native target architecture is X86
  Threads enabled.
  Doxygen disabled.
  Sphinx disabled.
  Constructing LLVMBuild project information
  CMake Error at cmake/modules/AddLLVM.cmake:259 (add_library):
    add_library cannot create target "LLVMSupport" because another target with
    the same name already exists.  The existing target is a static library
    created in source directory "A:/LLVM/llvm/source/lib/Support".  See
    documentation for policy CMP0002 for more details.
  Call Stack (most recent call first):
    cmake/modules/AddLLVM.cmake:352 (llvm_add_library)
    lib/Support/CMakeLists.txt:1 (add_llvm_library)
  
  
  CMake Error at cmake/modules/AddLLVM.cmake:360 (install):
    install TARGETS given target "LLVMSupport" which does not exist in this
    directory.
  Call Stack (most recent call first):
    lib/Support/CMakeLists.txt:1 (add_llvm_library)
  
  ...

http://reviews.llvm.org/D4177