Add Meson build system

Add build descriptions for the Meson build system.

I appreciate speex is in maintenance-only mode at this point, but it would be great if we could get this merged upstream.

It'd be very useful for projects such as GStreamer (and eventually VLC) who would easily be able to build speex as meson subproject.

We'd be happy to maintain this of course.

This meson build has been tested on Linux, on Windows with MSVC, macOS, macOS/iOS universal cross-builds, Android universal cross-builds, and mingw 32-bit/64-bit cross-builds as part of GStreamer's cerbero package build system. (ci pipeline)