FreeBSD prefers to call the C++ compiler c++ (just like Linux, Darwin, ..)
This is an upstreaming-request for cmake patches applied by FreeBSD.
Original request to default to c++ for the C++ compiler, instead of CC: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=203793
The change was added to FreeBSD's packaging of CMake with version 3.9: https://svnweb.freebsd.org/ports?view=revision&revision=449853
It'd be nice to have this upstream so that downstream CMake needs less patching. (In years gone by, there were supported FreeBSD releases with gcc or with clang, and this was kind of confused; now that every supported released version runs clang things are more simple).