bootstrap: don't reset config-h.in timestamp newer than configure

bootstrap: don't reset config-h.in timestamp newer than configure

Various autotools rules will compare the timestamps of config-h.in
and configure, and if config-h.in is newer than configure, attempt
to regenerate it.  This breaks `make distcheck` which we need to
produce new releases.  So update config-h.in timestamp, but only
up to the configure file.

The breakage is caught because distcheck makes source dirs read-only
before running.

$ make distcheck
...
make[3]: Entering directory '.../libtool-2.4.7.62-7132f/_build/sub'
  GEN      ../../libltdl/configure
autom4te-2.72: error: cannot create autom4te.cache in .../libtool-2.4.7.62-7132f/libltdl: Permission denied
make[3]: *** [Makefile:2411: ../../libltdl/configure] Error 1

* bootstrap.conf (libtool_fudge_timestamps): Set config-h.in timestamp
to configure.