Use @rpath as the `INSTALL_PATH` for the Static Library and Framework targets. closes #1682

For more details, see: http://www.dribin.org/dave/blog/archives/2009/11/15/rpath/ and https://developer.apple.com/library/mac/documentation/developertools/conceptual/DynamicLibraries/100-Articles/RunpathDependentLibraries.html