The current logic for CSTD/CXXSTD requires homogenity as far as the
supported C/C++ standards, which is a sensible default. However, when
dealing with differing versions of C++, some code may compile with C++11, but
not C++17 (for instance), so in order to avoid having people convert over their
code to the new standard, give the users the ability to specify the standard on
a per-program basis..
This will allow a user to override the supporting standard for a set of
programs, mixing C++11 with C++14 (for instance).
MFC after: 2 months
MFC with: r345706