Revert "cmake: use supplied RPATH instead of guessing"
This reverts commit 8ec5adc953d6bbabbe62d7f7229f146202704e53.
This broke stdenv on Darwin, see #15881. A Darwin user will need to fix
the problem and un-revert. This commit causes CMake to use the same
RPATH settings as the compiler, so it is obviously correct.