[libclc] Pass -fapprox-func when compiling 'native' builtins

[frasercrmck]

The libclc build system isn't well set up to pass arbitrary options to arbitrary source files in a non-intrusive way. There isn't currently any other motivating example to warrant rewriting the build system just to satisfy this requirement. So this commit uses a filename-based approach to inserting this option into the list of compile flags.

[arsenm]

Cmake already lets you do this:

set_source_files_properties(
  ${CMAKE_CURRENT_SOURCE_DIR}/native.cl
  PROPERTIES COMPILE_FLAGS "-fapprox-func")

[frasercrmck]

set_source_files_properties

That's true but since we're compiling with custom commands (I think) the properties won't automatically get picked up.

That said, we could perhaps set the properties as you suggest and retrieve them with get_source_file_property. We'd still need to set the properties "explicitly" as the SOURCES system isn't well set up to add arbitrary (lists of) stuff on top of the file name. But at least we could do it in the top-level CMakeLists which feels less hacky to me.

I'll play around with it.

[frasercrmck]

I've pushed an update with uses the COMPILE_OPTIONS property. It's definitely better, thanks for the suggestion.

It would be best if there were CMakeLists.txt in each sub-directory which added the properties. However, because we currently have this system where targets include from multiple directories (e.g., r600 includes stuff from r600/, amdgpu/ and generic/), and you can only add_subdirectory once, it gets a bit complicated. We'd have to ensure that generic is only included once, but that would also mean that it would be harder to do target-specific things in there. It might be possible (I think you can set a custom (target-specific?) build directory in add_subdirectory), but for now I'm just setting the properties in the top-level CMakeLists.txt. I'd be a bit nervous of rewriting the build system at this time.