llvm-rc preprocessing on non-windows host should add more c flags
On linux or mac clang-cl must know where windows sdk and vc include dir are. I add the flags(-imsvc ${MSVC_INCLUDE}
for example) in my toolchain file via CMAKE_#{LANG}_FLAGS. But preprocessing for llvm-rc does not pick up these flags and results in header files(winver.h
for example) not found.