Correctly add libc++abi to project list.

Closes #8.
Cleanup unused code.
Add support for FreeBSD.

Tested under FreeBSD 10.0-p11.
As on Linux, bundle libc++abi with libc++.
Fix hardcoded /usr prefix on FreeBSD.
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.
Merge remote-tracking branch 'matthias/topic/freebsd'

* matthias/topic/freebsd:
  Fix default linker search paths.
  Fix hardcoded /usr prefix on FreeBSD.
  As on Linux, bundle libc++abi with libc++.
  Add support for FreeBSD.
  Cleanup unused code.
  Correctly add libc++abi to project list.