* 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
