vala: fix VPATH builds
This change fixes automake bug#9859.
* automake.in (lang_vala_finish_target): Create the stamp file
'${derived}_vala.stamp' in $(srcdir), not in $(builddir).
Also, don't try to chdir to the $(srcdir) to trigger the rebuild
rules, since that is just wrong in a VPATH setup.
* tests/vala-vpath.test, tests/vala2.test: Extend to catch more
possible VPATH issues.
* tests/Makefile.am (XFAIL_TESTS): Remove 'vala-vpath.test'.
* NEWS, THANKS: Update.
Co-authored-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Copyright-paperwork-exempt: yes