[tlbuild] Bug in libpng affecting Solaris 10

[Karl Berry]

    mm> I'm attaching a patch for TL

I just (finally) installed it.  Thanks for your patience.

    (running autoreconf didn't change anything).

autoreconf is the right command, but it's necessary to modify
our libpng/configure.ac, not their libpng/libpng-src/configure.ac.

In a couple of cases, TL calls the original configure script, but
usually not.  -k