Vala: Use $(srcdir) in rebuild rules.

Vala: Use $(srcdir) in rebuild rules.

valac is always run in srcdir as the generated files are distributed.
So srcdir needs to be taken into account in the rebuild rules to not
wrongly trigger a rebuild in the wrong directory.

* automake.in (lang_vala_finish_target): Rebuild files in
$(srcdir).
* tests/vala2.test: Test rebuild rules from VPATH build.

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