Rework CXXSTD setting via r345708

This change allows the user to once again override the C++ standard, restoring
high-level pre-r345708 behavior.

This also unbreaks building lib/ofed/libibnetdisc/Makefile with a non-C++11
capable compiler, e.g., g++ 4.2.1, as the library supported being built with
older C++ standards.

MFC after: 2 weeks
MFC with: r345708
Reviewed by: emaste
Reported by: jbeich
Differential Revision: https://reviews.freebsd.org/D19895 (as part of a larger change)