To the extent that a package depends on particular c++ features, and not
just a standard level, using the c++ feature macros in our package
description format would be useful, and unambiguous.

I still feel that flags should be determined by the package manager in
context of a toolchain description, but the feature test macros would make
it easier to say no with explanation.

The pattern of taking flags unilaterally from packages doesn't compose
well. A package should not, for example say it turns on exceptions and
confuse the overall build, but simple tests to check if the toolchain
provides what the package is looking for would be useful.