Update civetweb.c

Updated to build clean in Visual Studio 2012 & 64 bit.  
The undef of _WIN32_WINNT is out dated at this point.

ws2tcpip.h needed to be defined in windows.

HAVE_POLL is defined in this new include, so we don't want to define it again.
Update civetweb.c

Added MSC_VER check to the include for ws2tcpip, as it broke the linux build.
Update civetweb.c

The define to include ws2tcpip was only looking to see if using ipv6, but this is a windows include so it should have checked for MSC_VER
Update civetweb.c

Update the defines to compile clean in Windows.  The last edit were not correct.
Merge pull request #20 from grenclave/patch-1

Update civetweb.c