m4: Fix AM_PROG_VALAC macro to take 3 parameters instead of 1

The AM_PROG_VALAC automake macro has been able to take 3 parameters (1
mandatory, 2 optional) for a while, just like AM_PATH_PYTHON.

See:
http://www.gnu.org/software/automake/manual/html_node/Vala-Support.html#index-AM_005fPROG_005fVALAC

https://bugzilla.gnome.org/show_bug.cgi?id=712361