[bndbuild] When a rule is deactivated by a constraint, bnduild should say it to the user when it want to build it

[rgiot]

For example, we have this rule

- tgt: checkdata
  phony: true
  constraint: Os(linux)
  cmd: extern python3 checkdata_analysis.py

It works properly under linux.
under windows it does not exists and the current message is

 bndbuild.exe checkdata
Failure
Target checkdata is not buildable. You may want to build clean

It is better if it becomes

 bndbuild.exe checkdata
Failure
Target checkdata is not buildable (it is deactivated by the constraint Os(linux)`. You may want to build clean