[mlpack] [mlpack/mlpack] Faster build times (#842)

[Ryan Curtin]

```
C:\projects\mlpack\src\mlpack\core\math\lin_alg.cpp(233): error C2065: 'M_SQRT2': undeclared identifier [C:\projects\mlpack\build\src\mlpack\mlpack.vcxproj]
```

That's our offending line, but it's strange because `<mlpack/prereqs.hpp>` includes `<cmath>`.

http://stackoverflow.com/questions/6563810/m-pi-works-with-math-h-but-not-with-cmath-in-visual-studio

So, worth a shot is the explanation that something else is including `<cmath>` before `_USE_MATH_DEFINES` is defined.  A "shot in the dark" idea is to push that `<cmath>` include and `_USE_MATH_DEFINES` definition in `prereqs.hpp` up to be the first thing included.  If you like, push a change like that and let's see what AppVeyor thinks... :)

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