Pluck options which are not applicable to C++ out of the C++ command line.

Pluck options which are not applicable to C++ out of the C++ command line.

-Wnested-externs
-W*-prototypes
-Wno-pointer-sign
-Wold-style-definition

Also, ignore -std=... for the moment. It's set globally to gnu99 now and
that leads to warnings in C++ compilation. We'll ponder other ways to cope
with this later.

This commit will allow us to raise WARNS further for C++.

Taken-from: FreeBSD