ethash should build with MSVC

[zv]

This patch allows ethash to build with MSVC under Windows.

I have built & installed the python package derived from this repository (pyethash) under Windows 10 and Fedora Linux without issue.

MSVC's /Wall does generate a number of informational compiler warnings, none of which appear new or appear serious.

[zv]

@chfast @karalabe - This library is still used by a variety of Python projects. Is there a current maintainer I can reach out to about getting something like this included?

[chfast]

What kind of warning?

[chfast]

BTW, I will be starting adding Python bindings to https://github.com/chfast/ethash.

[zv]

@chfast Regarding the build warnings, it's worth noting that MSVC at /Wall enables a wider variety of informational warnings than GCC's corresponding -Wall. In any case, here is the result of a verbose (pip -v) installation of pyethash under Windows 10

Your ethash implementation is looking great and many will gladly use your new implementation. However, it won't address the outstanding issue that many would like clean dependency fetching from pypi which is going to entail either updates to the pypi project or an effort to convince python packages which depend upon pyethash to switch to an API-compatible fork.