Remove the MK_LIBCPLUSPLUS option

This option has been equivalent to any form of C++ support since libstdc++
was removed. Therefore, replace all MK_LIBCPLUSPLUS uses with MK_CXX.

Reviewed By: emaste
Differential Revision: https://reviews.freebsd.org/D27974

(cherry picked from commit 5cf6f1c4bc281ac45ced086bc2e393f732f2fcc2)