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