A better method to require compiler features

[nia <nia%NetBSD.org@localhost>]

In light of recent discussions, I would like to add a
new variable to the pkgsrc infrastructure. It could
will be used in the following ways:

USE_CXX_FEATURES+=	c++14

To require at least GCC 7, but not NetBSD gcc 7:

USE_CXX_FEATURES+=	filesystem

To require at least GCC 4.9:

USE_CXX_FEATURES+=	regex

This would set the appropriate GCC_REQD without intefering
with the compiler wrappers.

In addition, users would be able to decide how missing
compiler features are handled.

setting in /etc/mk.conf:

SKIP_CXX_REQUIREMENTS=	yes

would skip building any packages where the current compiler is
"good enough". This is helpful for situations where vendored
or specifically modified toolchains are required, and upstream
GCC is not suitable.