Makes it easier to include glslang in a larger CMake project---instead
of having to call `target_link_libraries(glslang OSDependent OGLCompiler
HLSL)`, for example, you only need to call
`target_link_libraries(glslang)` and it will pull in the helpers it
needs.

This is also better in terms of cleaning up the "public interface",
of sorts, for building glslang: end-users probably shouldn't need to
know or be explicitly dependent on internal targets.