on --disable-doc, prevent meson from scanning doc directories

[dimpase]

sagelib's meson build already has an option to disable (very slow) scanning of doc directories;
here we connect this option to ./configure --disable-doc

To test, use ./configure --disable-doc and witness a speed-up in the beginning of sagelib build.
Namely,

[sagelib-10.9.beta2] [spkg-install]   Generating targets:   0%|          | 0/1681 eta ?
[sagelib-10.9.beta2] [spkg-install]   Generating targets:  10%|▉         | 162/1681 eta 00:00
[sagelib-10.9.beta2] [spkg-install]   Generating targets:  23%|██▎       | 386/1681 eta 00:00
[sagelib-10.9.beta2] [spkg-install]   Generating targets:  42%|████▏     | 702/1681 eta 00:00
[sagelib-10.9.beta2] [spkg-install]   Generating targets:  84%|████████▍ | 1412/1681 eta 00:00
[sagelib-10.9.beta2] [spkg-install]   Generating targets:  84%|████████▍ | 1412/1681 eta 00:00
[sagelib-10.9.beta2] [spkg-install]   Generating targets:  89%|████████▉ | 1494/1681 eta 00:04
[sagelib-10.9.beta2] [spkg-install]   Generating targets:  89%|████████▉ | 1495/1681 eta 00:04
[sagelib-10.9.beta2] [spkg-install]   Generating targets:  89%|████████▉ | 1495/1681 eta 00:04
[sagelib-10.9.beta2] [spkg-install]   Generating targets:  91%|█████████ | 1528/1681 eta 00:09
[sagelib-10.9.beta2] [spkg-install]   Generating targets:  91%|█████████ | 1529/1681 eta 00:10
[...]
[sagelib-10.9.beta2] [spkg-install]   Generating targets:  96%|█████████▌| 1617/1681 eta 02:27
[sagelib-10.9.beta2] [spkg-install]   Generating targets:  96%|█████████▋| 1618/1681 eta 02:21
[sagelib-10.9.beta2] [spkg-install]   Generating targets:  98%|█████████▊| 1646/1681 eta 00:39
[sagelib-10.9.beta2] [spkg-install]   Generating targets:  99%|█████████▉| 1666/1681 eta 00:12
[sagelib-10.9.beta2] [spkg-install]   Generating targets: 100%|█████████▉| 1680/1681 eta 00:00

should be much faster (and also the number of targets is smaller, about 1400)

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

[dimpase]

@nbruin - you recently complained about this.

[github-actions]

Documentation preview for this PR (built with commit 5ea6250; changes) is ready! 🎉
This preview will update shortly after each push to this PR.

[enriqueartal]

It works for me in Fedora 43.

[tobiasdiez]

Okay, if it works for people using sage-the-distro...

[nbruin]

How sticky is this option? Normally I just run "make -j8 build" which then may or may not rerun configure. I guess one would need to run ./configure --disable-doc to activate this option. Would it get lost if make decides configure should be rerun? Perhaps it's good to document somewhere how to activate it and what actions may deactivate it.

[dimpase]

How sticky is this option? Normally I just run "make -j8 build" which then may or may not rerun configure. I guess one would need to run ./configure --disable-doc to activate this option. Would it get lost if make decides configure should be rerun?

No. It is as sticky as any other "normal" ./configure option.
It is cached in ./config.status (you can print what's stored by ./config.status --config).

If ./configure is invoked from make, the same cached options are used.

To get docbuild back, you need to re-run ./configure without this option, or equivalently, with --enable-doc

Perhaps it's good to document somewhere how to activate it and what actions may deactivate it.

well, it's pretty generic autotools stuff.

[dimpase]

this option is not new, but when we changed to using meson it was not "reconnected" to the build system properly. Qua functionaly nothing has changed, qua efficiently it's a speedup.