Add support for non-installed libc++.

Sometimes (e.g., on continuous integration machines) we want to build with a
libc++ that hasn't been installed to /usr/lib/c++/v1. With this commit, we can
specify a LIBCXX_BUILD_DIR and LIBCXX_SOURCE_DIR to pick up headers and the
library itself from a source and build tree.
Enable cap_rights_t test on non-Capsicum platforms.

We don't actually need to compile this stuff: we're just testing the
interpretation of annotations.  That said, do conditionally compile things
when we're on a Capsicum-supporting platform (ahem) to give the compiler a
chance to complain about our use of the primitives.
Merge branch 'master' into working