RFR: 8348586: Optionally silence make warnings about non-control variables

[Magnus Ihse Bursie]

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](https://bugs.openjdk.org/browse/JDK-8347825), this happens whenever you run idea.sh, and it is confusing to users.

I introduce a general way of silencing such complaints using the new control variable `ALLOW`, and start by applying it on the idea.sh script.

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Commit messages:
 - 8348586: Optionally silence make warnings about non-control variables

Changes: https://git.openjdk.org/jdk/pull/23302/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23302&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348586
  Stats: 14 lines in 3 files changed: 6 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/23302.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23302/head:pull/23302

PR: https://git.openjdk.org/jdk/pull/23302