Aaaah ... is this happening after all of the tests have been run ?
Looking at this bug report (same problem, same module, but different subdir) it would seem so .... or (UPDATE) maybe not, on second look.

That bug report indicates a couple of workarounds:
1) change the "test:" line to "test test_dynamic:". I assume this pertains to the generated Makefile in the particular subdir (PNG/libpng/Makefile in your case);
2) try installing a different version of ExtUtils::MakeMaker.

BUT, if the tests have already been run, and the results of those tests are satisfactory to you, then I would just run 'make install' and not worry about this problem.

Cheers,
Rob