Two things to stop `make distcheck' from trying to rebuild

Two things to stop `make distcheck' from trying to rebuild
distributed files (m4/ltversion.m4 and config/ltmain.in) in the
readonly source tree:

* Makefile.am (stamp-vcl): Keep only one copy of stamp-vcl; in
$(top_srcdir), so that an up-to-date version is rolled into the
dist tarball even from a VPATH build.
($(top_srcdir)/config/ltmain.sh): Don't depend on Makefile,
otherwise ltmain.sh needs to be rebuilt after every config.status
run, and then configure needs to be rebuilt, and it all goes
horribly wrong :-/  Unfortunately, we can't depend on Makefile.am
either, because that is a circular dependency.  The downside of
this change is that it is now possible to edit Makefile.am to
change the way ltmain.sh is generated, and the dependency
information can't tell that ltmain.sh needs regenerating.