We just have to suppress these warnings: Mingw's math.h uses gcc's
__builtin_choose_expr() facility to declare isnan, isfinite, and
signbit. But as implemented in at least some versions of gcc,
__builtin_choose_expr() can generate type warnings even from
branches that are not taken.
Fixes bug 22801; bugfix on 0.2.8.1-alpha.
Nick Mathewson