[boost] [concept_check] Merge changes to master

Hi, I merged some old changes to master, all warning fixes, all old enough to be fully tested on trunk: https://github.com/boostorg/concept_check/commit/229e3055a03210da4dd10d43bc5... https://github.com/boostorg/concept_check/commit/ccd9f52467953f4b569f52cb6a6... https://github.com/boostorg/concept_check/commit/9521991561bf574772ce48d38c2... https://github.com/boostorg/concept_check/commit/168e60aa3d5238cd25b341661a6... I cherry picked the first two because they're a bit old. It's a bit late now, but do you think I should have done this as a pull request? Daniel