vala: avoid potential useless remakes (minor bugfix)

vala: avoid potential useless remakes (minor bugfix)

* automake.in (lang_vala_finish_target): Ensure the timestamp file
from which the C files generated from Vala sources depend on gets
created with a modification time that is truly older than those of
said generated C files.  This prevents make from attempting useless
rebuilds (which were bound to happen deterministically on systems
with sub-second timestamp resolutions).  It is worth noting that,
luckily, those useless rebuild ended up being a no-op, since the
Vala compiler is careful not to update the timestamp of an output
file if its content has not changed from the previous version.
Still, the useless rebuilds messed up "make -q" and "make -n"
invocations, and were technically incorrect (despite being, as
noted, inoffensive in practice).

Problem revealed by failure of tests vala-mix.test and vala5.test
on a fast Solaris 10 system whose filesystem had a sub-second
timestamp resolution.