Don't list meta/meta-version.h in mutter_built_sources

It's already listed in libmutterinclude_built_headers.

https://bugzilla.gnome.org/show_bug.cgi?id=734960
Don't remove meta/meta-version.h during make clean

meta/meta-version.h is generated by configure, so it should be removed
with make distclean, not make clean. This is the heuristic described in
https://www.gnu.org/software/automake/manual/html_node/Clean.html

Cleaning it in make clean means that configure; make clean; make won't
work properly as meta/meta-version.h will be removed and not
regenerated.

https://bugzilla.gnome.org/show_bug.cgi?id=734960