problems compiling on osx with bazel

[cpietsch]

I ran into some errors compiling guetzli with bazel on osx 10.11.
These changes to WORKSPACE fixed it for me:

1. change http://github.com/glennrp/libpng/archive/v1.2.57.zip to https://github.com/glennrp/libpng/archive/v1.2.57.zip
2. change third_party/butteraugli/ to the absolute path e.g. /Users/chris/guetzli/third_party/butteraugli/

[robryk]

Could you provide the errors that you were getting before you made those changes?

[cpietsch]

INFO: Elapsed time: 1,618s
java.lang.RuntimeException: Unrecoverable error while evaluating node 'REPOSITORY_DIRECTORY:@png_archive' (requested by nodes 'REPOSITORY:@png_archive')
	at com.google.devtools.build.skyframe.ParallelEvaluator$Evaluate.run(ParallelEvaluator.java:1003)

ERROR: /Users/chris/Documents/projekte/guetzli/guetzli/BUILD:4:1: no such package '@butteraugli//': local_repository rule //external:butteraugli must specify an existing directory and referenced by '//:guetzli_lib'.
ERROR: Loading failed; build aborted.