Provided a default value for SAGE_ROOT that can be overridden by an environment variable or configuration file

[Krishnadubey1008]

This PR fixes #39870
added following code to env.py

DEFAULT_SAGE_ROOT = "" 
SAGE_ROOT = var("SAGE_ROOT", DEFAULT_SAGE_ROOT)

to ensure that SAGE_ROOT is not hardcoded and can be dynamically set or overridden

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[tobiasdiez]

Thanks!

The fallback

sage/src/sage/meson.build

Line 7 in 871ba9d

conf_data.set('SAGE_ROOT', meson.current_source_dir() / '..' / '..')

also needs to be removed (which might lead to other problems that then need to be fixed).

[tobiasdiez]

I think you can simply inline this variable.

[antonio-rojas]

As mentioned in #39870 (comment), SAGE_ROOT must be unset by default, not set to the empty string. Otherwise, in the next two lines in env.py

SAGE_SRC = var("SAGE_SRC", join(SAGE_ROOT, "src"), SAGE_LIB)
SAGE_DOC_SRC = var("SAGE_DOC_SRC", join(SAGE_ROOT, "src", "doc"), SAGE_DOC)

the variables are assigned the wrong fallback (involving SAGE_ROOT) instead of the correct one (the last one). So just removing the fallback in config.py like Tobias suggested in #39885 (review) should suffice.

[Krishnadubey1008]

@antonio-rojas @tobiasdiez thanks for your help, I had applied the changes please review

[antonio-rojas]

I still think that config.py is a distro thing that should be installed by the distro (be it sage-the-distro or something else), not by sagelib. But at least it with this it won't break downstreams.