update libtool.m4 to silence autotools warnings

"warning: AC_LANG_CONFTEST: no AC_LANG_SOURCE call detected in body"

Running autogen.sh resulted in many more changes, so just update
one of the files.

See http://git.savannah.gnu.org/cgit/libtool.git/commit/?id=2680f79c22
for the upstream fix that silences the warnings.