Minor fixups for Vala support.

Minor fixups for Vala support.

* automake.in: $(VALAFLAGS) comes after $(AM_VALAFLAGS).
* doc/automake.texi (Vala Support): Add some references,
document AM_VALAFLAGS, streamline a bit.
* lib/am/vala.am: Copyright blurb.
* m4/vala.m4 (AM_PROG_VALAC): Rewrite using AS_VERSION_COMPARE.
* tests/vala.test: Fix minor nits.
* tests/vala1.test: Likewise.
* tests/vala2.test: Likewise.
* tests/vala3.test: Likewise.
* tests/vala4.test: New test, for version argument of
AM_PROG_VALAC.
* tests/Makefile.am: Update.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>