configure: log libpng-config details and fix libs setup

This way we can see what script it tried to use as well as the flags
it got back from that script.

This also fixes the libs lookup to use --ldflags instead of --libs
as the former includes -L flags.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

configure.ac

@@ -133,11 +133,13 @@ yes|"")
   ])
   ;;
 *)
+  AC_MSG_CHECKING([libpng-config script])
   LIBPNG_CONFIG=$with_png/bin/libpng-config
   if test -e "$LIBPNG_CONFIG"; then
     LIBPNG_CFLAGS=`$LIBPNG_CONFIG --cflags`
-    LIBPNG_LIBS=`$LIBPNG_CONFIG --libs`
+    LIBPNG_LIBS=`$LIBPNG_CONFIG --ldflags`
     with_png=yes
+    AC_MSG_RESULT([$LIBPNG_CONFIG, cflags: $LIBPNG_CFLAGS, libs: $LIBPNG_LIBS])
   else
     AC_MSG_ERROR([png support requested, but not found at requested location: $LIBPNG_CONFIG])
   fi