Swift: support `-rpath` on Darwin
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.
Backport: release
Topic-rename: swift-rpath-darwin
Edited by Brad King