* EbSvtAv1Dec.h: remove prototype for non-existent functions
Those functions have apparently never existed in the tree:
- eb_peek_sequence_header()
- eb_svt_decode_obu()
- eb_svt_decode_tu()
- eb_dec_flush()
- eb_get_stream_info()
* EbSvtAv1Dec.h: consistently use the svt_av1_dec_ namespace for public functions
* EbSvtAv1Enc.h: consistently use the svt_av1_enc_ namespace for public functions
* Make Unix conditionals the fallback
Provides a clear path what needs to be ported if the system is neither
Windows nor POSIX-like. If the difference from Linux is minor like on
macOS or BSDs then new conditionals can be added on case-by-case basis.
* gstreamer-plugin: sync Meson version check with CMake
* gstreamer-plugin: don't assume CMake can find libs by default