Add OGG-FLAC support

[ghost]

As I stated in #18, I had a work-in-progress fork that adds OGG-FLAC support to this library. I believe it is now ready to be pulled and integrated into the main project.

I've tested building and running this code on a simple instance that passes an OGG-FLAC stream to this version of libshout, and streams seem to work well.

Caveats:

• a new m4 file must be added under the m4 folder - it can be found here as flac.m4, but I couldn't include it in this PR, since the m4 folder is itself a submodule (presumably to the original Gitlab repo)
• there are problems with timeouts when streaming long segments of silence, like with at least one another codec as pointed out by @rillian here, and codec_flac here doesn't do manual page flushes. I'm not sure if it might be worth pointing out somewhere
• In private test use, I've heard reports of some media players (such as VLC) having issues due to discontinuities over timestamps after song changes (the timestamps get reset to zero on song change, which causes VLC to spam "timestamp conversion failed"), but it's not been tested if this also happens on other OGG streams and could equally well be the fault of the program that actually feeds the stream to libshout