Link Foundation/CoreServices into unit tests so they build with DISABLE_WX.
Don't murder the default PATH and CMAKE_SYSTEM_PREFIX_PATH on OS X.  Just prioritize /usr.

Changing PATH basically screws everything up, and while the attempt to
avoid MacPorts copies of system libraries was well-intentioned, it made
the OS X buildbot unable to pick up ffmpeg and libusb.

It's sufficient to put /usr first to make sure we use the system copies
of duplicated libraries.
Merge pull request #1435 from comex/osx-build

OS X CMake improvements