Consult libpng-config more consistently.

This is mainly for simplicity, but it should also avoid
some future problems like the ones we recently had with NetBSD.
* configure.ac (LIBPNG): Configure after LIBZ.  Use libpng-config
for cflags, too.  Append -lz if we're not already doing that with
LIBZ.  Do not bother appending -lm, since we always append that.
Coalesce some duplicate code.
* src/Makefile.in (PNG_CFLAGS): New var.
(ALL_CFLAGS): Use it.
* src/image.c [HAVE_PNG]: Don't worry about <libpng/png.h>, as
CFLAGS now handles this.

Fixes: debbugs:17339

1 files changed, 0 insertions, 4 deletions

diff --git a/src/image.c b/src/image.c
index d558540c6e7..c26c0db2b4f 100644
--- a/src/image.c
+++ b/src/image.c
@@ -22,12 +22,8 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 #include <unistd.h>
 
 #ifdef HAVE_PNG
-#if defined HAVE_LIBPNG_PNG_H
-# include <libpng/png.h>
-#else
 # include <png.h>
 #endif
-#endif
 
 #include <setjmp.h>
 #include <c-ctype.h>