>From 52d34fbc84c1d87a41c435a0db9494d768ebc667 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Mon, 12 Feb 2018 16:39:56 -0800
Subject: [PATCH] Work if libpng is in /usr/local (2nd try)

Problem reported by Werner Lemberg in:
https://lists.gnu.org/r/emacs-devel/2018-02/msg00304.html
with a further fix suggested by Glenn Morris in Bug#30431#14.
* configure.ac: Try pkg-config before libpng-config.
Adjust LIBPNG accordingly, and append -lz regardless of
whether it was pkg-config.
---
 configure.ac | 75 ++++++++++++++++++++++++++++++++++--------------------------
 1 file changed, 42 insertions(+), 33 deletions(-)

diff --git a/configure.ac b/configure.ac
index f2a8332d71..cb452e053b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -3612,39 +3612,48 @@ AC_DEFUN
   if test "$opsys" = mingw32; then
     AC_CHECK_HEADER([png.h], [HAVE_PNG=yes])
   elif test "${HAVE_X11}" = "yes" || test "${HAVE_W32}" = "yes"; then
-    AC_MSG_CHECKING([for png])
-    png_cflags=`(libpng-config --cflags) 2>&AS_MESSAGE_LOG_FD` &&
-    png_ldflags=`(libpng-config --ldflags) 2>&AS_MESSAGE_LOG_FD` || {
-      # libpng-config does not work; configure by hand.
-      # Debian unstable as of July 2003 has multiple libpngs, and puts png.h
-      # in /usr/include/libpng.
-      if test -r /usr/include/libpng/png.h &&
-	 test ! -r /usr/include/png.h; then
-	png_cflags=-I/usr/include/libpng
-      else
-	png_cflags=
-      fi
-      png_ldflags='-lpng'
-    }
-    SAVE_CFLAGS=$CFLAGS
-    SAVE_LIBS=$LIBS
-    CFLAGS="$CFLAGS $png_cflags"
-    LIBS="$png_ldflags -lz -lm $LIBS"
-    AC_LINK_IFELSE(
-      [AC_LANG_PROGRAM([[#include <png.h>]],
-	 [[return !png_get_channels (0, 0);]])],
-      [HAVE_PNG=yes
-       PNG_CFLAGS=`AS_ECHO(["$png_cflags"]) | sed -e "$edit_cflags"`
-       LIBPNG=$png_ldflags
-       # $LIBPNG requires explicit -lz in some cases.
-       # We don't know what those cases are, exactly, so play it safe and
-       # append -lz to any nonempty $LIBPNG, unless we're already using LIBZ.
-       if test -n "$LIBPNG" && test -z "$LIBZ"; then
-	 LIBPNG="$LIBPNG -lz"
-       fi])
-    CFLAGS=$SAVE_CFLAGS
-    LIBS=$SAVE_LIBS
-    AC_MSG_RESULT([$HAVE_PNG])
+    EMACS_CHECK_MODULES([PNG], [libpng >= 1.0.0])
+    if test $HAVE_PNG = yes; then
+      LIBPNG=$PNG_LIBS
+    else
+      # Test old way in case pkg-config doesn't have it (older machines).
+      AC_MSG_CHECKING([for libpng not configured by pkg-config])
+
+      png_cflags=`(libpng-config --cflags) 2>&AS_MESSAGE_LOG_FD` &&
+      png_ldflags=`(libpng-config --ldflags) 2>&AS_MESSAGE_LOG_FD` || {
+	# libpng-config does not work; configure by hand.
+	# Debian unstable as of July 2003 has multiple libpngs, and puts png.h
+	# in /usr/include/libpng.
+	if test -r /usr/include/libpng/png.h &&
+	   test ! -r /usr/include/png.h; then
+	  png_cflags=-I/usr/include/libpng
+	else
+	  png_cflags=
+	fi
+	png_ldflags='-lpng'
+      }
+      SAVE_CFLAGS=$CFLAGS
+      SAVE_LIBS=$LIBS
+      CFLAGS="$CFLAGS $png_cflags"
+      LIBS="$png_ldflags -lz -lm $LIBS"
+      AC_LINK_IFELSE(
+	[AC_LANG_PROGRAM([[#include <png.h>]],
+	   [[return !png_get_channels (0, 0);]])],
+	[HAVE_PNG=yes
+	 PNG_CFLAGS=`AS_ECHO(["$png_cflags"]) | sed -e "$edit_cflags"`
+	 LIBPNG=$png_ldflags])
+      CFLAGS=$SAVE_CFLAGS
+      LIBS=$SAVE_LIBS
+      AC_MSG_RESULT([$HAVE_PNG])
+    fi
+
+    # $LIBPNG requires explicit -lz in some cases.
+    # We don't know what those cases are, exactly, so play it safe and
+    # append -lz to any nonempty $LIBPNG, unless we're already using LIBZ.
+    case " $LIBPNG ",$LIBZ in
+      *' -lz '*, | *' ',?*) ;;
+      *) LIBPNG="$LIBPNG -lz" ;;
+    esac
   fi
 fi
 if test $HAVE_PNG = yes; then
-- 
2.14.3