Howto: Easily adapt packages that use USE_LANGUAGES

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

Hey, there's been a few packages that have been incorrectly modified
to use USE_(CC|CXX)_FEATURES, so I thought I'd post a short guide on
adapting packages.

First of all, do a fresh build with USE_LANGAUGES set to simply
"c c++", or unset if the package doesn't use C++.

grep work/.work.log for std=. If it contains a value of std=,
set USE_CC_FEATURES or USE_CXX_FEATURES to that. Congratulations,
you're done.

If it doesn't, take the previous year-specific value of USE_LANGAUGES,
and add that to FORCE_C_STD/FORCE_CXX_STD *and*
USE_CC_FEATURES/USE_CXX_FEATURES. You're done!