MinGW: core.stdc.math macro implementations.
MinGW: Use __mingw_* versions of C formatting functions.

The unprefixed versions are pulled in from the Microsoft C
runtime by default, and thus do not support long doubles
and, in the case of ldexp, the hexadecimal floating point
literal syntax.
Merge pull request #481 from klickverbot/mingw

Floating point math/formatting for MinGW