Add FLAC support without changing raylib source code

These changes should build raylib with -DSUPPORT_FILEFORMAT_FLAC=1.

This enables FLAC support as mentioned in #78.

I tested both TARGET_LINUX and TARGET_WIN64_MINGW and a sample FLAC file just works on both.

Notes:

Maybe the flag could be passed only to the raudio.c module build step. But I don't know if that definition changes anything else and may be needed for all modules anyway.
Maybe this could be done in a different way. I didn't want to assume too much and insert unrequested complexity, so a little more thought could shape this support introduction better than this bare minimum idea.
I don't think an additional configure option for the repo's build is needed (in order to have this be flexible/optional).
- For now, the available configure options (hotreloading, bundled come to mind) seem fare more relevant than what this would offer.
I guess it mostly comes down to the downsides of turning this support on.
- Don't know much about that, but may need to be looked into.
- For now I can say that on my system the binary size increased:
- from 5174kB to 5548kB, so ~7% (on TARGET_LINUX)
- from 5723kB to 6085kB, so ~6% (on TARGET_WIN64_MINGW)

tsoding / musializer