When you set a variable on the make command line that is not a known control variable, make emits a note like this:

Note: Command line contains non-control variables:
* MODULES=
* IDEA_OUTPUT=/mnt/h/openjdk/git-jdk2/.idea
Make sure it is not mistyped, and that you intend to override this variable.
'make help' will list known control variables.

Since JDK-8347825, this happens whenever you run idea.sh, and it is confusing to users.