Issue title: Error when trying to compile with shaderc for metal on macos
 Issue body: I'm trying to compile some bgfx shaders using shaderc and bgfx.cmake through a cmake script. When compiling on windows I don't have any issues compiling to dx11 or glsl or anything like that, but on macOS there's an issue when compiling to metal. The full error is available [here](https://pastebin.com/raw/08MkGVc7).

It kinda seems like it doesn't understand the syntax at all which is really weird (it complains about every single line of the shader), and it also explains about HLSL parsing for some reason: 
(92): error at column 93, HLSL parsing failed.
ERROR: 7 compilation errors.  No code generated.

Any help would be appreciated. The relevant CMake code is available [here](https://pastebin.com/y6dsQp7L).

 Comments: 
 Comment 0: There is no platform `metal`:

>       --platform <platform>     Target platform.
>          android
>          asm.js
>          ios
>          linux
>          orbis
>          osx
>          windows
 Comment 1: Thanks for answering! For some clarification though, the function can be run like this on windows:
```
if(WIN32)
    compile_shader(voxel_game ${PROJECT_SOURCE_DIR}/src/shader_src/vs_cubes.sc vertex dx11 vs_5_0)
    compile_shader(voxel_game ${PROJECT_SOURCE_DIR}/src/shader_src/fs_cubes.sc fragment dx11 ps_5_0)
endif()
```
And that works perfectly. Here the SHADER_PLATFORM argument is dx11 which is not part of the --platform alternatives either, so I don't really think that's the issue.
 Comment 2: Nevermind, that actually fixed it on mac: I just needed to change the path so that it looked in the metal folder for the shader. It's interesting though that it's not an issue on windows, but since it's working I guess I'll just leave it. Thanks!
{'source': 'GithubIssues', 'repo_name_with_owner': 'bkaradzic/bgfx', 'issue_id': 1067099245, 'repo_stars': 12857.0}