Add LIB_CXX so that C++ libraries will use CXX to link.

This fixes C++ libraries not implicitly linking in libc++. This is
generally not an issue because the final linking with the compiled binary
will involve CXX via PROG_CXX or other means. It is however
inconsistent with libraries implicitly linking in libc and problematic
for trying to build libraries with '-z defs' to ensure all direct
dependencies are linked in.

libatf-c++ is currently the only consumer of this new feature.

Differential Revision: https://reviews.freebsd.org/D2039
Reviewed by: imp
Discussed with: bapt
MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division