CMake improvements

[moien007]

This is PR #137 (which I messed up, my bad) plus new changes.

Changes are:

• Upgrading from CMake 2.6 to 3.4.3
• Linking against ws2_32 and winmm if compiler is MSVC
• Conditionally defining ENET_DLL macro
• Exporting all symbols for Win32 so that a .def file or __declspec(dllexport) is not required

[Croydon]

@moien007 Not sure if you are still motivated since you did not get an official response, but could you perhaps rebase this? Thanks!

[moien007]

@moien007 Not sure if you are still motivated since you did not get an official response, but could you perhaps rebase this? Thanks!

Solved the merge conflict; is that what you meant?

[Croydon]

Yes, thanks.

@lsalzman Could you perhaps have a look at this patch? It isn't big so it shouldn't take long to review