Add Meson build system

[HolyWu]

No description provided.

[dwbuiten]

I don't think FFMS2 is looking to move to meson at this point in time, it has a lot of teething issues at the moment (too many hardcoded assumptions on e.g. windows, broken stuff like builtin checking, the requirement to install tools to run configure (because gotta use meson master), putting the host arch in the target path, not using /lib by default).

This also doesn't seem like it would work with MSVC (one of the main reasons people like meson sometimes), and doesn't integrate tests or various checks, or have uninstal support.

We can maybe revisit once there is an actual advantage to using it over autotools, which are working just fine right now, and are not a maitenece burden.