On Mac OS there is no rpath to set for the executable to locate
libraries. Instead, dylibs themselves have an install_name which is the
path where executables can load them from (added to the executable at
link time).

Bug swiftshader:68

Change-Id: Ia72eba0076ec83fc4492a5c118b4f5420d03a640
Reviewed-on: https://swiftshader-review.googlesource.com/10208Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <capn@google.com>