Provide a way of building the package on Windows sytem without MSYS installed.

MSYS requirement to build network package on Windows is a burden to end users. After reading a [proposal](http://www.haskell.org/pipermail/libraries/2014-August/023458.html) motivated by this problem, I took some time to implement a proof-of-concept system of installation for Windows users without MSYS.

The idea is to run the build with MSYS and then extract the options passed to GHC as well as created headers file into a standalone build that can be executed without MSYS.

This is likely not in a shape to be included in a master repo but it may be a start of the feature described above.
Isn't there a more reliable detection? I.e. is `MSYSTEM` set if-and-only-if the environment will be able to run `./configure`?