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

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