Fix building on MinGW64 Windows

Mingw64 provides mstcpip.h and the build fails (redefinition) if the struct tcp_keepalive is redefined. Do not define the struct if __MINGW64__ is defined. Note that I had to manually pass the compile definition to cmake: -D__MINGW64__=1
Merge pull request #941 from veegee/master

Fix building on MinGW64 Windows