libc++: Install headers in include/, rather than lib/

When using the configure build system, the libc++ headers were being
installed in lib/, whereas cmake installs them in include/. Since
include/ makes more sense for headers, we'll make both systems install
headers there.
Darwin: Look for libc++ headers in include/, rather than lib/

Up until now we were expecting that when libc++ is installed alongside
clang the headers would be in lib/, which was true if the configure
build was used and false if the cmake build was.

We've now corrected the configure build to install in include/, and
with this change we'll be able to find the correct headers with both
build systems.