[llvm] r201921 - Use relative rpath so that the installation and build dirs are relocatable.

[Rafael Espíndola]

> For 1) everything used to Just Work™, but is broken now. I'm going to
> make CMAKE_INSTALL_NAME_DIR in projects/compiler-rt/lib/sanitizer
> point to the libraries' absolute paths to fix that, if you do not
> object.

Not at all, since that is what was being done before.

> For 2) we don't want to force our users to install the runtime into
> the system, because it's still under development. Instead the users
> are supposed to ship their binaries with the appropriate version of
> the runtime lib, and they anyway have to fix the runtime library using
> install_name_tool, so nothing changes for them with your change.

OK, if using install_name_tool is expected, setting the
CMAKE_INSTALL_NAME_DIR is probably the correct thing to do.

BTW, how is this handled in the configure+make build? We should
probably keep them in sync.

Cheers,
Rafael