fix: use unix path separators in makefiles

[wpbonelli]

There is a conversion to Unix path separators on L1349 but then calling relpath() restores Windows separators again. Add another conversion so generated makefiles consistently use Unix path separators, even when generated on Windows.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.130%. Comparing base (6d55d09) to head (0c6a07a).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@              Coverage Diff              @@
##           develop      #271       +/-   ##
=============================================
+ Coverage   80.073%   80.130%   +0.056%     
=============================================
  Files           20        20               
  Lines         3523      3523               
=============================================
+ Hits          2821      2823        +2     
+ Misses         702       700        -2     

Files with missing lines	Coverage Δ
pymake/pymake_base.py	85.971% <100.000%> (ø)

... and 1 file with indirect coverage changes

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