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, 8 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index 3d6cc362b67..d9837bc3c12 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,13 @@
 2014-05-04  Paul Eggert  <eggert@cs.ucla.edu>
 
+        Consult libpng-config more consistently (Bug#17339).
+        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.
+
         * autogen.sh: Use ‘"’ to quote a message that often contains ‘'’.
 
         Require ImageMagick >= 6.3.5, due to PixelSetMagickColor (Bug#17339).