On OSX both backends are built. Generally we want to use the cocoa
backend by default and in case it is not supported, i.e because
the application is not using a bundle then we should fallback
to the gtk one.
Those files got renamed to .c to work around an automake issue, but
Meson needs them to have .m extension. Better rename them at build time
in Makefile.am since that's where the workaround is needed.
https://bugzilla.gnome.org/show_bug.cgi?id=672777
