The ability to cross-compile glib got broken after the
merge of the 'signal-performance' branch as the assumption
was made that the generated glib-genmarshal can be executed
on the host (which isn't valid when cross-compiling).
Fixed this by using the just-built glib-genmarshal for normal
compilations and the native (host) glib-genmarshal when doing a
cross-compilation as was also done in several other areas of GLib
Tested for host=x86_64-unknown-linux-gnu, target=x86_64-unknown-linux-gnu
and host=x86_64-unknown-linux-gnu, target=i686-w64-mingw32
https://bugzilla.gnome.org/show_bug.cgi?id=671676