[Mac] Building a plugin with the distributed binary SDK looks for MacOS10.13.sdk for Accelerate.framework

@aseth1 reported that building a plugin on Mac with the 4.1 expects a MacOS10.13.sdk for linking to Accelerate.framework using an absolute path. We should change this to a relative path somehow.

This is actually a Simbody issue, because it's Simbody that links to Accelerate.framework.