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, 2 insertions, 0 deletions

diff --git a/src/Makefile.in b/src/Makefile.in
index 388923596c9..c35e38bb290 100644
--- a/src/Makefile.in
+++ b/src/Makefile.in
@@ -77,6 +77,7 @@ C_SWITCH_MACHINE=@C_SWITCH_MACHINE@
 C_SWITCH_SYSTEM=@C_SWITCH_SYSTEM@
 
 GNUSTEP_CFLAGS=@GNUSTEP_CFLAGS@
+PNG_CFLAGS=@PNG_CFLAGS@
 
 ## Define C_SWITCH_X_SITE to contain any special flags your compiler
 ## may need to deal with X Windows.  For instance, if you've defined
@@ -325,6 +326,7 @@ ALL_CFLAGS=-Demacs $(MYCPPFLAGS) -I. -I$(srcdir) \
   -I$(lib) -I$(srcdir)/../lib \
   $(C_SWITCH_MACHINE) $(C_SWITCH_SYSTEM) $(C_SWITCH_X_SITE) \
   $(GNUSTEP_CFLAGS) $(CFLAGS_SOUND) $(RSVG_CFLAGS) $(IMAGEMAGICK_CFLAGS) \
+  $(PNG_CFLAGS) \
   $(LIBXML2_CFLAGS) $(DBUS_CFLAGS) $(XRANDR_CFLAGS) $(XINERAMA_CFLAGS) \
   $(SETTINGS_CFLAGS) $(FREETYPE_CFLAGS) $(FONTCONFIG_CFLAGS) \
   $(LIBOTF_CFLAGS) $(M17N_FLT_CFLAGS) $(DEPFLAGS) \