Switch to libpng-waf.
Merge branch 'libpng' into develop
Integrate ext/*.

Integrate ext/gmock-waf 0eec1f8.
Integrate ext/libsfz 79d8f33.
Integrate ext/libzipxx 8f6c541.
Integrate ext/waf-sfiera 3362af4.
Default to clang.
Merge branch 'integrate' into develop