Added preliminary fenv.h support on Windows.

This update includes an fenv.h implementation for Windows x64, as
well as some improvements to the the C99 math support layer for
MSVC as well. Changes include:

- Addition of cfenv.hpp wrapper header for fenv.h support.
- Exposure of #pragma STDC FENV_ACCESS equivalent, if supported,
  via the UP_FENV_ACCESS macro.
- Optimized the uniform_int_distribution template implementation
  for cases when the random engine's result type is exactly half
  the size of the uniform_int_distribution's result_type.
- Updated the MSVS project generation script with better support
  for assembly file custom build steps and module definition files.
- Improved the strtof/strtod/strtold/etc. implementation.