MinGW32 Compatibility Fixes.

Here is a collection of fixes that allows the develop branch of Poco to build on Windows 7/8 with with MinGW32.

Unlike VS, MinGW / GCC still requires users to call `Poco::Net::initializeNetwork()`.  In previous Poco versions this was called from within various DNS functions, etc.  I would be curious to know if there is a way to automatically initialize winsock with MinGW -- but I can't seem to figure out a method to do this with GCC.

This fix makes several modifications to Makefiles to include `Net.cpp`.  Additionally a step to compile `pocomsg.mc` via `windmc` is added.

This PR is the result of our efforts to upgrade Poco in openFrameworks -- we are currently on version 1.4.3p and are looking forward to getting 1.5.3+ into the core!  So far, all other platforms have compiled successfully.  Here's the issue if you are curious https://github.com/openframeworks/openFrameworks/pull/3116.

Thanks for Poco!