[cmake] Restrict resource file usage to Windows build hosts

[smeenai]

Summary

Resource file compilation requires a working resource compiler.
Unfortunately, llvm-rc isn't quite there yet [1], and cmake's rc
invocation only works on Windows [2]. Until both those issues are
addressed, disable resource file usage on non-Windows build hosts, to
unblock Windows cross-compilation. This is also consistent with the
existing comment, which says "If *on Windows* and building with MSVC".

[1] http://lists.llvm.org/pipermail/llvm-dev/2017-October/118433.html
[2] https://public.kitware.com/pipermail/cmake/2017-October/066441.html

cmake/modules/AddLLVM.cmake

 
 # If on Windows and building with MSVC, add the resource script containing the
 # VERSIONINFO data to the project.  This embeds version resource information
 # into the output .exe or .dll.
 # TODO: Enable for MinGW Windows builds too.
 #
 function(add_windows_version_resource_file OUT_VAR)
   set(sources ${ARGN})
-  if (MSVC)
+  if (MSVC AND CMAKE_HOST_SYSTEM_NAME STREQUAL "Windows")
     set(resource_file ${LLVM_SOURCE_DIR}/resources/windows_version_resource.rc)
     if(EXISTS ${resource_file})
       set(sources ${sources} ${resource_file})
       source_group("Resource Files" ${resource_file})
       set(windows_resource_file ${resource_file} PARENT_SCOPE)
     endif()
-  endif(MSVC)
+  endif(MSVC AND CMAKE_HOST_SYSTEM_NAME STREQUAL "Windows")
 
   set(${OUT_VAR} ${sources} PARENT_SCOPE)
 endfunction(add_windows_version_resource_file)
 
 # set_windows_version_resource_properties(name resource_file...
 #   VERSION_MAJOR int
 #     Optional major version number (defaults to LLVM_VERSION_MAJOR)
 #   VERSION_MINOR int