The stamp-vcl file rules caused non-gnu make programs to rerun

The stamp-vcl file rules caused non-gnu make programs to rerun
parts of the configuration process on each invocation.  Instead
of relying on a stamp file, we now compare version numbers of
generated files with the ChangeLog timestamp manually in the
affected make rules to decide whether regenerating is necessary:

* Makefile.am (EXTRA_DIST): Remove stamp-vcl.
(clean-ltmain-sh): Also remove users' obsolescent stamp-vcl.
(rebuild): Keep some common code here for...
(libtool, libltdl/m4/ltversion.m4, libltdl/config/ltmain.sh):
Rewritten to work without reference to intermediate stamp-vcl
file.
* configure.ac (CONFIG_STATUS_DEPENDENCIES): Change from
stamp-vcl to ChangeLog.
* NEWS: Removed entry saying this fix is needed.