Set CMAKE_SHARED_LIBRARY_SUFFIX to always create .so files.

[KeaneWong]

Description

Currently, the way that CMake creates the library files always generates .dylib files on Mac. Compilation will work fine, but attempting to import in any Python app will cause the import to fail, as the interpreter expects .so files for library files.
For some reason, using the flag
--cmake-args " -DCMAKE_SHARED_LIBRARY_SUFFIX=.so"
does not fix the issue. Additionally, setting the variable before the project() line in the CMakeLists also remains inneffective. So, the workaround is to add
set(CMAKE_SHARED_LIBRARY_SUFFIX .so)
right after the project() line.

@Mergifyio backports 2.1.x

Fixes #206

Contributor Checklist

• Commit messages follow the project guidelines.
• Tests that thoroughly check the new feature have been added/Regression tests checking the bug and its fix have been added; the added tests pass locally
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• Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
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• Check contributor checklist is correct.
• Check CI results: changes do not issue any warning.
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