configury: simplify calculation of module file extension.

* configure.ac (LT_SYS_MODULE_EXT): Use this...
(mfour_shrext): Remove.  ...instead of working it out manually.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
modules: Use LT_MODULE_EXT for host module file extension.

* m4/module.c (m4__module_open): Use LT_MODULE_EXT instead of
no longer calculated SYS_SHREXT.
* m4/path.c (FILE_SUFFIXES): Likewise.

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