How to get clang with OpenMP working on MSVC 2015 - c++

Question

I try to get clang 5.0.0 working for Visual Studio 2015, because I need OpenMP 3.0 features. I installed the clang compiler (not the vs2015 version which does not have any openmp support) and use cmake:
cmake_minimum_required(VERSION 2.8.10)
project(myproject)
find_package(OpenMP)
if (OPENMP_FOUND)
set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}")
set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
endif()
include_directories("include")
add_library(libFoo STATIC Foo.cpp)
install(TARGETS Foo libFoo LIBRARY DESTINATION lib ARCHIVE DESTINATION lib)
When I now try to configure a MSVC 14 2015 Win64 build with or without toolchain LLVM-vs2014 I always get an error, that OpenMP is not found:
The C compiler identification is Clang 5.0.0
The CXX compiler identification is Clang 5.0.0
Check for working C compiler: D:/Program Files/LLVM/msbuild-bin/cl.exe
Check for working C compiler: D:/Program Files/LLVM/msbuild-bin/cl.exe -- works
Detecting C compiler ABI info
Detecting C compiler ABI info - done
Detecting C compile features
Detecting C compile features - done
Check for working CXX compiler: D:/Program Files/LLVM/msbuild-bin/cl.exe
Check for working CXX compiler: D:/Program Files/LLVM/msbuild-bin/cl.exe -- works
Detecting CXX compiler ABI info
Detecting CXX compiler ABI info - done
Detecting CXX compile features
Detecting CXX compile features - done
Could NOT find OpenMP_C (missing: OpenMP_C_FLAGS OpenMP_C_LIB_NAMES) (found version "1.0")
Could NOT find OpenMP_CXX (missing: OpenMP_CXX_FLAGS OpenMP_CXX_LIB_NAMES) (found version "1.0")
Configuring done
The used compiler seems to be the right one (Installed clang, not the Microsoft version), it autodetects the clang-cl binary, but OpenMP fails.
I tried to manually specify the compilers with "specify native compilers" and get the same result. It even selects the clang-cl version instead of clang++.
Related Answer, which does not solve the problem:
Compiling C code with openmp using clang-cl - Recent clang has libomp.lib and libiomp5.dll included

Answer 1

It is really a bit tricky and the cmake autodetection doesn't seem to work very well. What helped is
OpenMP_CXX_FLAGS="-Xclang -fopenmp"
OpenMP_C_FLAGS="-Xclang -fopenmp"
And making sure that libomp.lib is in the link libraries.
-Xclang tells the clang binary, that the following options are in clang format and not in MSVC format and -fopenmp is then just the usual OpenMP flag. Setting it any other way did not work, though.
The general pattern is: -Xclang -clangFlag1 -Xclang -clangFlag2.... Namely each Clang Style flag requires its own -Xclang.