Saleem Abdulrasool authored Nov 17, 2019 and Brad King committed Nov 18, 2019

Darwin also has the concept of RPATH.  Additionally, the flag is
identical to that on other Unixish platforms.  Simply avoid the `-rpath`
handling on Windows.

This enables the use of `BUILD_WITH_INSTALL_RPATH` and `INSTALL_RPATH`
with Swift targets on Darwin.