Fix make distcheck

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