[cmake] explicit feature=OFF should have hierarchical priority over cached or explicit builtin_feature=ON v2

[ferdymercury]

This Pull request:

Changes or fixes:

This v2 of #18413

Fixes https://its.cern.ch/jira/browse/ROOT-10743

Related:

https://its.cern.ch/jira/browse/ROOT-9385

Checklist:

• tested changes locally
• updated the docs (if necessary)

[ferdymercury]

@pcanal I have tried to follow your suggestion.

My question now is, how do I prevent enabling fftw3=ON if I passed explicitly -Dfftw3=OFF ?

Right now the sequence

cmake -Dbuiltin_fftw3=ON ../root_src/
cmake -Dfftw3=OFF ../root_src/

still leaves fftw3 ON.

Do I need to do something like https://stackoverflow.com/a/54302498/7471760 or rather https://stackoverflow.com/a/61986176/7471760?

Sth like:

if (NOT fftw3 and NOT DEFINED CACHE{builtin_fftw3})
   set(fftw3 CACHE BOOL ON FORCE)
endif()

[pcanal]

Try without the FORCE ...

[github-actions]

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Results for commit 8998e58.

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[ferdymercury]

Try without the FORCE ...

I tried and it does not solve the issue. Probably because the default option fftw3=OFF, so that has priority over the cache?

So without force -Dbuiltin_fftw3=ON does not turn fftw3=ON.

I tried also NOT DEFINED CACHE, to no avail.

It seems that this post is relevant:
https://stackoverflow.com/questions/43542381/set-a-cmake-variable-if-it-is-not-changed-by-the-user

But then we are back to the debate... is this extra complexity worth or can we stick with v1 #18413?