Fix bug #7716 with PNG image support libraries on Windows.

 src/image.c <Qlibpng_version>: New variable.
 (syms_of_image): Intern and staticpro it.  Set its value to the
 version of PNG library we were compiled with.
 (my_png_error, png_load): Avoid GCC warnings about direct access
 to png_ptr->jmpbuf.
 lisp/term/w32-win.el (image-library-alist): Set up correctly for
 libpng versions both before and after 1.4.0.
 admin/nt/README.W32: Update the information about PNG support libraries.
 nt/INSTALL: Update the information about PNG support libraries.

1 files changed, 16 insertions, 8 deletions

diff --git a/admin/nt/README.W32 b/admin/nt/README.W32
index 9f1df8abd3..b8d2610b65 100644
--- a/admin/nt/README.W32
+++ b/admin/nt/README.W32
@@ -119,14 +119,19 @@ See the end of the file for license conditions.
   libXpm library from X11R7.3.
 
   Emacs can also support some other image formats with appropriate
-  libraries.  These libraries are all available as part of GTK, or from
-  gnuwin32.sourceforge.net.  Emacs will find them if the directory they
-  are installed in is on the PATH.
-
-      PNG: requires the PNG reference library 1.2 or later, which will
-      be named libpng13d.dll, libpng13.dll, libpng12d.dll, libpng12.dll
-      or libpng.dll.  LibPNG requires zlib, which should come from the same
-      source as you got libpng.
+  libraries.  These libraries are all available as part of GTK
+  download for Windows (http://www.gtk.org/download-windows.html), or
+  from the GnuWin32 project.  Emacs will find them if the directory
+  they are installed in is on the PATH.
+
+      PNG: requires the PNG reference library 1.4 or later, which will
+      be named libpng14.dll or libpng14-14.dll.  LibPNG requires zlib,
+      which should come from the same source as you got libpng.
+      Starting with Emacs 23.3, the precompiled Emacs binaries are
+      built with libpng 1.4.x and later, and are incompatible with
+      earlier versions of libpng DLLs.  So if you have libpng 1.2.x,
+      the PNG support will not work, and you will have to download
+      newer versions.
 
       JPEG: requires the Independant JPEG Group's libjpeg 6b or later,
       which will be called jpeg62.dll, libjpeg.dll, jpeg-62.dll or jpeg.dll.
@@ -137,6 +142,9 @@ See the end of the file for license conditions.
       GIF: requires libungif or giflib 4.1 or later, which will be
       called giflib4.dll, libungif4.dll or libungif.dll.
 
+   If you have image support DLLs under different names, customize the
+   value of `image-library-alist'.
+
    In addition, Emacs can be compiled to support SVG.  This precompiled
    distribution has not been compiled that way, since the SVG library
    or one or more of its extensive dependencies appear to be