Log Message:
openssl: Unbreak Darwin installs after --libdir change.

Adding --libdir seems innocuous enough, but due to OpenSSL's crazy complicated
build system actually results in LIBDIR becoming unset, which trickles down to
the Darwin -install_name being wrong.  This broke the 'openssl' command and
thus many many dependencies.

Switch to $(libdir) instead which appears to still be set correctly.