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>

configure.ac

@@ -2741,11 +2741,11 @@ AC_ARG_ENABLE(png,
 if test "${enable_png}" != "no"; then
 AC_CHECK_HEADERS(png.h, [
   VLC_SAVE_FLAGS
-  LDFLAGS="${LDFLAGS} -lz"
+  LDFLAGS="${LDFLAGS} -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] )
   VLC_RESTORE_FLAGS
   ])
 fi