These are here to prevent linker errors, since `gcontenttype.[ch]`
aren’t compiled on Windows or macOS.
The implementations are stubs to be filled out by someone who knows each
platform, at some point in the future.
Signed-off-by: Philip Withnall <withnall@endlessm.com>
Fixes: #1791
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
