make distcheck was unhappy for various reasons:
* headers aren't data, so use _HEADERS otherwise compilation fails
* Mark the gir & typelib data as cleanfiles so they aren't left around
after make clean
* Don't nuke the .la file. This breaks make uninstall, leave it up to
distributions to not install .la files if they don't want them.
https://bugzilla.gnome.org/show_bug.cgi?id=705850
