signbit macro doesn't compile under gcc -Werror=strict-aliasing -O2
The definition of the signbit macro from math_c99.h uses a type-punned pointer, which breaks strict-aliasing rules and will not compile on gcc with -O2 and the -Werror=strict-aliasing option.
From what I've seen, other implementations type pun via a union instead, which is standards compliant.
Test program: https://gist.github.com/1616683
Downstream issue: https://github.com/rubinius/rubinius/issues/1515
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