Work even if libpng is installed in /usr/local

Problem reported by Werner Lemberg in:
https://lists.gnu.org/r/emacs-devel/2018-02/msg00304.html
* configure.ac: Try pkg-config before libpng-config.

diff --git a/configure.ac b/configure.ac
index f2a8332d71a3cedcdab90395d35ffcae5a3dd5c4..ec25e9b8c77e7dff6beecbb734278e4caabeeb9a 100644 (file)
--- a/configure.ac
+++ b/configure.ac
@@ -3612,7 +3612,11 @@ elif test "${with_png}" != no; then
   if test "$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])
+   EMACS_CHECK_MODULES([PNG], [libpng >= 1.0.0])
+   if test $HAVE_PNG = no; then
+    # Test old way in case pkg-config doesn't have it (older machines).
+    AC_MSG_CHECKING([for libpng not configured by pkg-config])
+
     png_cflags=`(libpng-config --cflags) 2>&AS_MESSAGE_LOG_FD` &&
     png_ldflags=`(libpng-config --ldflags) 2>&AS_MESSAGE_LOG_FD` || {
       # libpng-config does not work; configure by hand.
@@ -3645,6 +3649,7 @@ elif test "${with_png}" != no; then
     CFLAGS=$SAVE_CFLAGS
     LIBS=$SAVE_LIBS
     AC_MSG_RESULT([$HAVE_PNG])
+   fi
   fi
 fi
 if test $HAVE_PNG = yes; then