Fix libpng checking on OpenBSD

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

configure.ac

@@ -2814,11 +2814,11 @@ AC_ARG_ENABLE(png,
   [  --enable-png            PNG support (default enabled)])
 if test "${enable_png}" != "no"; then
 AC_CHECK_HEADERS(png.h, [
-  LDFLAGS="${LDFLAGS_save} -lz"
+  LDFLAGS="${LDFLAGS_save} -lz $LIBM"
   AC_CHECK_LIB(png, png_set_rows, [
-    VLC_ADD_LIBS([png],[-lpng -lz -lm])
+    VLC_ADD_LIBS([png],[-lpng -lz $LIBM])
     VLC_ADD_PLUGIN([png osdmenu osd_parser])],
-    [],[-lz])
+    [],[-lz $LIBM])
     LDFLAGS="${LDFLAGS_save}"
   ])
 fi