try for system libmgba first

[alucryd]

Patch made for the arch linux package, figured I would try to upstream it. Please let me know if that is something desirable, and if so whether it's in acceptable state or requires modifications.

[Pokechu22]

Based on #9600 (comment) there were previously concerns about distros shipping libmbga but not libmgba-dev, and also about determinism with netplay/movies where the same version of dolphin should imply the same version of libmgba. I'm not sure how relevant these concerns (particularly the former) are nowadays though.

[delroth]

Based on #9600 (comment) there were previously concerns about distros shipping libmbga but not libmgba-dev

I don't think you'd have a pkg-config file if it's not libmgba-dev anyway? So it will fall back to Externals as it currently does.

determinism with netplay/movies where the same version of dolphin should imply the same version of libmgba

Sounds like we should fix this by including a libmgba version string in the netplay handshake then?

[v1993]

Note: this PR had an issue with include directory detection that's fixed by #11566.