This fixes the libpng check on OpenBSD. Although the code was passing libm to
ultimately link the library the test itself was not and thus failing. Also at
Remi's request I changed the existing and newly introduced use of -lm to $LIBM.
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>
(cherry picked from commit 900041eb4d79eadc51a32c879fbedfa6262be767)

Conflicts:

	configure.ac