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

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.