pngcp: tool to copy PNG files

This adds pngcp to the build together with a pngcp.dfa configuration test; the
test revealed some configuration bugs which are fixed by corrections to the
_SUPPORTED macros.

pngcp builds on all tested configurations and a number of bugs have been fixed
to make this happen relative to the version in libpng 1.7 contrib/examples.
pngcp.dfa will have to be different for 1.7 but pngcp.c should work fine (not
yet tested).  pngcp itself is still missing a usage message; this is a
preliminary version, although since it behaves the same way as 'cp' most unoids
shouldn't have a problem using it correctly.

Signed-off-by: John Bowler <jbowler@acm.org>

contrib/libtests/pngimage.c

@@ -1082,6 +1082,7 @@ compare_read(struct display *dp, int applied_transforms)
       }
 
       else
+#     ifdef PNG_sBIT_SUPPORTED
       {
          unsigned long y;
          int bpp;   /* bits-per-pixel then bytes-per-pixel */
@@ -1243,6 +1244,10 @@ compare_read(struct display *dp, int applied_transforms)
             }
          } /* for y */
       }
+#     else /* !sBIT */
+         display_log(dp, INTERNAL_ERROR,
+               "active shift transform but no sBIT support");
+#     endif /* !sBIT */
    }
 
    return 1; /* compare succeeded */