Do not build libatf-c++ when WITHOUT_CXX

libatf-c++ requires C++ support.

From jrtc27: bit slightly odd this isn't gated by MK_TESTS (which itself
depends on MK_CXX), but this makes sense given the current behaviour.

Reported by: Michael Dexter, Build Option Survey
Reviewed by: imp, jrtc27
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32732