Version macros in C API

[Zipdox2]

Version 6.0 broke compatibility with my C code because of an enum change. I wanted to use a macro to make my code compatible with both the old and the new version but apparently the version macros RTAUDIO_VERSION_MAJOR, RTAUDIO_VERSION_MINOR, etc are only available in the C++ API. The only solution I can think of now is using the build system to detect the version and supply a macro to the compiler.

I would prefer that the C version also gets version macros.