fix: use classic extension handling unless otherwise requested

[henryiii]

WITH_SOABI is only defined in CMake 3.17+, and it doesn't really quite do the expected thing for PyPy (being fixed on the PyPy side). For now, let's use the classic method for adding the extension unless a user requests otherwise.

[wjakob]

Regarding the test failure: maybe there is a difference between SO and EXT_SUFFIX after all? Looks great other than that!

[henryiii]

It has to work with SO, otherwise the classic mode would be broken. The problem was that the function runs in the calling scope, so it doesn't "capture" variables when it was defined (CMake needs a C++11 capture specifier ;) ). When using add_subdirectory, you are in a scope where the variable wasn't defined, causing a completely empty extension.