add -shared suffix extension to allow building newest e.g. zip.so

[henderkes]

What does this PR do?

WIP

Checklist before merging

If your PR involves the changes mentioned below and completed the action, please tick the corresponding option.
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• If you modified *.php or *.json, run them locally to ensure your changes are valid:
  • composer cs-fix
  • composer analyse
  • composer test
  • bin/spc dev:sort-config
• If it's an extension or dependency update, please ensure the following:
  • Add your test combination to src/globals/test-extensions.php.
  • If adding new or fixing bugs, add commit message containing extension test or test extensions to trigger full test suite.

[henderkes]

see #911

[henderkes]

Maybe simpler idea: extract to php-src/zip/spc-shared and if the directory exists, build from there in Extension.php. Not sure if this would also override $this->source_dir, though.

[henderkes]

The only frequently changing version of these is zip. Maybe instead of doing something special here, we can just override the buildshared method of zip.php.