MinGW-w64 Compatibility with v2.10.1

[Hoyt, David]

A build using the latest from MinGW-w64 fails with a redefinition error b/c it 
finds socklen_t in two places. Configure finds both winsock2.h and ws2tcipip.h 
and when it finds winsock.h, it does a "typedef int socklen_t" in 
gl/sys_socket.in.h but this is already defined in ws2tcpip.h. When I specified 
-D_WIN32_WINNT=0x0501 for Windows XP or higher support only, it compiled 
cleanly. It looks like a guard is needed at line 154 of gl/sys_socket.in.h - 
something along the lines of (this has not been tested): 

#ifndef @HAVE_WS2TCPIP_H@
typedef int socklen_t;
#endif

Thanks,
- David Hoyt