Initial support for the vala programming language.

Initial support for the vala programming language.

* automake.in: Add %known_libraries, lang_vala_rewrite,
lang_vala_finish and lang_vala_target_hook to support the Vala
programming language. Register Vala language hooks.
* lib/am/vala.am: Empty rules file to prevent creation of depend2
based rules for Vala code.
* lib/am/Makefile.am (dist_am_DATA): Add vala.am.
* m4/vala.m4: Provide AC_PROG_VALAC for detecting the Vala compiler.
* m4/Makefile.am (dist_m4data_DATA): Add vala.m4.
* tests/vala.test: Test Vala support.
* tests/Makefile.am: Update.

Signed-off-by: Mathias Hasselmann <mathias.hasselmann@gmx.de>
Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>