Development changes

Previously, when building the ``matplotlib._png`` extension, the build
script would add "png" and "z" to the extensions ``.libraries`` attribute (if
pkg-config information is not available, which is in particular the case on
Windows).

In particular, this implies that the Windows build would look up files named
``png.lib`` and ``z.lib``; but neither libpng upstream nor zlib upstream
provides these files by default.  (On Linux, this would look up ``libpng.so``
and ``libz.so``, which are indeed standard names.)

Instead, on Windows, we now look up ``libpng16.lib`` and ``zlib.lib``, which
*are* the upstream names for the shared libraries (as of libpng 1.6.x).

For a statically-linked build, the upstream names are ``libpng16_static.lib``
and ``zlibstatic.lib``; one still needs to manually rename them if such a build
is desired.

Packaging DLLs