Fixes #1 - Corrected mingw issue in string.cc (check __MINGW__)

This fix is pretty self-explanatory. I've included stdlib.h to ensure that
__MINGW__ is defined, and check if this is defined in addition to testing if
we're building on MSVC.

This will probably still be broken on Cygwin, but everyone hates Cygwin so it
might not be much of an issue. Still, simply testing if we're building on
Windows is probably good enough.