After this change, SolveSpace does not contain nonfree assets.
Additionally, Perl is not required for the build.
Note that in the US, case law suggests that copyright does
not apply to bitmap fonts:
http://www.renpy.org/wiki/renpy/misc/Bitmap_Fonts_and_Copyright
Nevertheless, it was prudent to replace the asset with something
that is unambiguously free.
As a side effect, zlib and libpng are now git submodules,
based on their respective official git repositories.
This is necessary, because MinGW has a different ABI and
it cannot use the prebuilt binaries built by MSVC.
The submodules are also used for Windows, for several reasons:
* to allow 64-bit builds;
* to allow using newer MSVC, which doesn't like the prebuilt
libraries;
* to keep the libraries updated.
whitequark