Fix default linker search paths.

The FreeBSD toolchain hardcoded /usr/lib as primary search path. If
we link against libc++, ld always choses /usr/lib/libc++.so instead of
our own version from PREFIX/lib. (The libc++ from /usr/lib also pulled
in libcxxrt.)

Moreover, we also add -rpath=PREFIX/lib as internal option to obviate
the need for setting LD_LIBRARY_PATH.