PR-108294 soname bump for modula2 runtime libraries.

The soname for the Modula-2 runtime libraries for the compiler based on
GCC 12 is 17 and didn't change for GCC 13.  This patch bumps the version
to 18.

libgm2/ChangeLog:

	* configure: Regenerate.
	* configure.ac (libtool_VERSION): Bump to 18:0:0.

Signed-off-by:  <gaiusmod2@gmail.com>