Log Message:
emulators/libretro-mgba: Handle _POSIX_C_SOURCE and extensions properly.

A _POSIX_C_SOURCE definition is needed for some locale definitions, but
some non-standard functions are also used (notably futimes). Avoid using
a global definition for _POSIX_C_SOURCE and define it where necessary,
while also defining _BSD_SOURCE and _NETBSD_SOURCE to get the right
prototype for futimes where necessary.