modules: use only host system module file extension in searches.

* configure.ac (SYS_SHREXT): Define to host system module file
extension.
* m4/module.c (suffixes), m4/path.c (FILE_SUFFIXES): Use it
instead of listing suffixes for all supported host systems.
Reported by Pavel Raiskup

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
configury: use modern LIBADD_DLOPEN over undocumented LIBADD_DL.

* Makefile.am (m4_libm4_la_LIBADD): Use modern LIBADD_DLOPEN
instead of obsolete and undocumented LIBADD_DL.

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