[libpng16] Fixed cexcept.h in which GCC 5 now reports that one of the auto

variables in the Try macro needs to be volatile to prevent value
being lost over the setjmp.

contrib/libtests/pngvalid.c

@@ -4097,7 +4097,7 @@ make_error(png_store* volatile psIn, png_byte PNG_CONST colour_type,
 
    Try
    {
-      png_structp pp;
+      volatile png_structp pp;
       png_infop pi;
 
       pp = set_store_for_write(ps, &pi, name);