build: compare `revision' rather than `correctver' in Makefile.am.

build: compare `revision' rather than `correctver' in Makefile.am.

* Makefile.am (rebuild): Set the shell variable `revision' rather
than `correctver' for clarity of purpose.
(bootstrap_edit, libltdl/config/ltmain.sh): Adjust.
(libltdl/m4/ltversion.m4): Likewise, and alse, instead of munging
the serial number comment line with set, extract `macro_version'
from this file, and compare it directly with `revision'.
(libtool): Likewise for `package_revision'.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>