It's also an option not to use dtoa.c:  Python still has fallback code that uses the OS double <-> char* conversions for the case where the configuration step can't figure out how to change the FPU control word.  In that case compilation should still succeed, and the resulting Python would show:

>>> import sys
>>> sys.float_repr_style
'legacy'

If there are tests failing with the 'legacy' mode, that may just indicate buggy tests that haven't been properly marked as depending on the short float repr.  (E.g., by decorating with "@unittest.skipUnless(getattr(sys, 'float_repr_style', '') == 'short'"), or poorly-designed tests that could be rewritten.