Fix libpymatching not using -fPIC (position independent code)

[codecov-commenter]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (e528e3a) 100.00% compared to head (956b174) 100.00%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #79 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 3 3 Lines 331 331 ========================================= Hits 331 331 ```

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