Add scons options for compilation verbosity

[clemisch]

(essentially the same as diffpy/libobjcryst#32)

Currently the manual compilation with scons dev is quite noisy: it prints all compiler commands and all compiler warnings. There is no flag to control it.

For development I found that annoying, mainly because it's hard to spot an error in the wall of warnings.

This PR adds scons flags verbose: true|false = false and warnigns: all|default|none = none. verbose controls if compiler commands are printed. warnings controls compiler warnings. For the latter:

• all: inherited -W flags (from python-config) + -Wextra
• default: no explicit warning-control flags
• none: suppress all warnings (only show errors) [new default]

I think it's more common to show warnings on demand rather than by default, but that's kinda subjective and I'm happy to change the default to warnings=all and verbose=true. That would mimick current behavior.

[github-actions]

Warning! No news item is found for this PR. If this is a user-facing
change/feature/fix,
please add a news item by copying the format from news/TEMPLATE.rst.
For best practices, please visit
https://scikit-package.github.io/scikit-package/snippets/news-file-format.html#how-do-i-create-a-news-file.

[clemisch]

I'll add the news item after we discussed the rest and came to an agreement ;-)

[vincefn]

If we move towards nanobind, the proposed branch switched to cmake, so it may supersede this. And it would remove libobjcryst. I guess that needs to be settled first.

[clemisch]

Yeah but how soon will that be, realistically?