We are now always checking which files are present or not, even when using our
internal copy of OMX, rather than hardcoding the ones present in it.
https://bugzilla.gnome.org/show_bug.cgi?id=792043
It seems cleaner to use the proper meson tools to include this path
rather than manually tweak the build flags.
This also allows us to simplify the OMX extensions detection code. We
are now always checking which files are present, even when using our
internal copy of OMX, rather than hardcoding the ones present in it.
https://bugzilla.gnome.org/show_bug.cgi?id=792043