[issue26121] Use C99 functions in math if available

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

There are accuracy issues with tgamma() and lgamma() on Windows. But less than on OS X.

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FAIL: test_mtestfile (test.test_math.MathTests)
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Traceback (most recent call last):
  File "C:\projects\cpython\lib\test\test_math.py", line 1287, in test_mtestfile
    '\n  '.join(failures))
AssertionError: Failures in test_mtestfile:
  lgam0085: lgamma(-99.99999999999999): expected -331.85460524980596, got -331.86198661322277 (error = 0.00738 (129854318490 ulps); permitted error = 1e-15 or 5 ulps)
  gam0085: gamma(-99.99999999999999): expected 7.540083334884096e-145, got 7.484632144060724e-145 (error = 5.55e-147 (38979707393612 ulps); permitted error = 0 or 20 ulps)
  gam0124: gamma(-176.5): expected -1.196e-321, got 0.0 (error = 1.2e-321 (243 ulps); permitted error = 0 or 20 ulps)
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Does this mean that we should not use libc implementations of tgamma() and lgamma() on Windows?

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue26121>
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