Lower required Automake version
Silence creation of gnome-music script, and update only if changed

Target gnome-music became a phony target because the dependency on Makefile
is in a separate line. As a result, it is always created on `make all'.
Silence compilation of the schema, and compile only if changed

The schema is always created on `make all' because the recipe is on the
all-local target. Move it to a separate target instead.
Merge pull request #136 from kyoushuu/lower-automake-required

Lower required Automake version, silence custom targets, and build them only if changed