Try an additional workaround for GLib inline issue.

It seems there are a range of GLib versions which #define G_INLINE_FUNC
unconditionally and thus we cannot work around the headers reliance on
obsolete GNU inline semantics by predefining this macro.

On these versions, we can define G_IMPLEMENT_INLINES to disable inline
function usage altogether.  This is less ideal but it will at least
allow compilation to proceed.