vala: AM_PROG_VALAC should not produce an error for tool-old valac

vala: AM_PROG_VALAC should not produce an error for tool-old valac

This change fixes automake bug#12688.

In the AM_PROG_VALAC macro, when the optional parameter specifying the
minimum release number is not provided, and/or if the Vala compiler is
not found, then there is a _warning_ message.

On the other hand, when the version number is specified and if the Vala
compiler is too old, there is an _error_ message.

This error message is problematic, because for a tarball, the Vala
compiler is not required: the generated C code is included in the
tarball.  So if a user wants to compile the software, he shouldn't
need the valac program with the right version.

* m4/vala.m4 (AM_PROG_VALAC): Modify to use AC_MSG_WARN instead
of AC_MSG_ERROR.
* t/vala4.sh: Adjust and enhance.
* doc/automake.texi (Vala Support): Likewise.
* THANKS: Update.

Co-authored-by: Matthieu Baerts <matttbe@glx-dock.org>
Co-authored-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Copyright-paperwork-exempt: yes
Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>