fix cmake error when configuring libcxxabi

[sebpop]

Summary

The problem occurs when configuring libcxxabi on x86_64 linux with:
$ cmake -DLLVM_PATH=$SRCTOP/llvm -DLIBCXXABI_LIBCXX_PATH=$SRCTOP/libcxx $SRCTOP/libcxxabi
[...]
CMake Error at llvm/cmake/modules/CheckCompilerVersion.cmake:31 (check_cxx_source_compiles):

Unknown CMake command "check_cxx_source_compiles".

Call Stack (most recent call first):

llvm/cmake/modules/HandleLLVMOptions.cmake:9 (include)
CMakeLists.txt:61 (include)

The fix is to include the cmake file that defines check_cxx_source_compiles.

[mclow.lists]

You're specifying -DLIBCXXABI_LIBCXX_PATH, but the Cmake bit that you're changing is inside of a elseif(NOT LLVM_ENABLE_LIBCXX) block.

Maybe I'm misunderstanding what's going on here (since my CMake-fu is close to nonexistent), but this seems wrong to me.

[EricWF]

This seems to have already been fixed. Sorry for the delay and thank you for the patch.

llvm/cmake/modules/CheckCompilerVersion.cmake

elseif(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
       elseif(NOT LLVM_ENABLE_LIBCXX)
         # Otherwise, test that we aren't using too old of a version of libstdc++
         # with the Clang compiler. This is tricky as there is no real way to
         # check the version of libstdc++ directly. Instead we test for a known
         # bug in libstdc++4.6 that is fixed in libstdc++4.7.
         set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
         set(OLD_CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES})
         set(CMAKE_REQUIRED_FLAGS "-std=c++0x")
+        INCLUDE(CheckCXXSourceCompiles)
         check_cxx_source_compiles("
 #include <atomic>
 std::atomic<float> x(0.0f);
 int main() { return (float)x; }"
           LLVM_NO_OLD_LIBSTDCXX)
         if(NOT LLVM_NO_OLD_LIBSTDCXX)
           message(FATAL_ERROR "Host Clang must be able to find libstdc++4.7 or newer!")
         endif()