libpng: update to 1.6.53

[aeiouaeiouaeiouaeiouaeiouaeiou]

Tested on

macOS 10.8.5 12F2560 x86_64
Xcode 5.1.1 5B1008

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• checked your Portfile with port lint?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?

[macportsbot]

Notifying maintainers:
@ryandesign for port libpng.