Hmm. That's funny. I can build Skia on my Mac with the toolchain provided by XCode 7.0.1. I also do not have /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/usr/lib/libz.dylib. I do have several suspiciously simliar looking .tbd files (libz.1.1.3.tbd, libz.1.2.5.tbd, libz.1.tbd, libz.tbd, etc.) Looking at one of the files, it looks like a .tbd file is instructions for some tool to install the corresponding .dylib file into /usr/lib.
I think my (working) build is picking up libz.dylib from /usr/lib (where I have all the .dylib versions of those .tbd files from the other directory). What do you have under /usr/lib? I vaguely recall there may be some Install-Command-Line-Tools step you need to do in the XCode UI before all these system libraries are put into place.