Building libpng with new zlib

[radarhere]

This PR adds a function that is intended to be build_libpng, but using the zlib generated by build_new_zlib. I imagine you'll have a better idea, but hopefully this is enough to start things going.

If you're interested, the reason that I'm creating this PR is because this version of pillow-wheels config.sh fails in Travis, but this version of pillow-wheels config.sh passes in Travis.

[matthew-brett]

I don't understand the error - can you say more about why it's happening? It appears that freetype2 and jpg_2000 aren't getting linked, but I can't see any indication why not, apart from this cryptic message : https://travis-ci.org/radarhere/pillow-wheels/jobs/289411067#L754 . I guess it's something to do with the difference between system zlib and compiled zlib on Mac, given that your fix works, but I'd like to understand it better.

[radarhere]

In my investigating, I looked at https://travis-ci.org/radarhere/pillow-wheels/jobs/289411067#L336 -

Undefined symbols for architecture i386:
  "_inflateValidate", referenced from:
      _png_inflate_claim in pngrutil.o
ld: symbol(s) not found for architecture i386

inflateValidate was added in zlib 1.2.9 - https://github.com/madler/zlib/blob/master/contrib/vstudio/vc10/zlibvc.def#L145

OS X 10.10 is the version being tested in Travis. Without knowing exactly which zlib version is on OS X by default, 10.10.5 was released on 13 August 2015. zlib 1.2.9 was released on 31 Dec 2016 - https://zlib.net/ChangeLog.txt

[matthew-brett]

Sorry to be a bit slow to get back to this one - do you mean that, the libpng build is not picking up the new libz? Should /usr/local/lib then be $BUILD_PREFIX ? Maybe that should be there by default? Can you add a test in tests/test_library_builders.sh ?

[radarhere]

In these comments, you'll have to ignore failures from 'MB_PYTHON_VERSION=2.7 PLAT=i686' - they are failing due to an unrelated issue.

Changing it from LDFLAGS="-L/usr/local/lib"

• to LDFLAGS="-L$BUILD_PREFIX/lib" works - https://travis-ci.org/radarhere/pillow-wheels/builds/297434635
• to LDFLAGS="-L$BUILD_PREFIX" does not - https://travis-ci.org/radarhere/pillow-wheels/builds/297438605

I've updated the commit for this PR. See what you think.

[ghost]

This commit is somewhat helpful: MacPython/matplotlib-wheels@0c76cd8. Perhaps this should be fixed for OSX globally?