Improve Vala support.

Improve Vala support.

* automake.in: Update to Vala 0.7.0 semantics, recover from removal
of built sources, support silent-rules, drop _PKGNAME variables
* doc/automake.texi, NEWS: Update.
* lib/am/vala.am: Bump copyright years.
* m4/vala.m4: Likewise.
* tests/vala.test: Remove obsolete grep tests.
* tests/vala1.test: Likewise.
* tests/vala2.test: Test full build, distcheck, and distclean.
* tests/vala3.test: Likewise, also test with subdir-objects.
* tests/vala4.test: Bump copyright years.
* tests/vala5.test: New test for per-target flags, expected to fail.
* tests/Makefile.am: Update.
* THANKS: Update.

Signed-off-by: Jürg Billeter <j@bitron.ch>
Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>