Add wxUSE_WINSOCK2 wxMSW option to include winsock2.h

Including <winsock.h> and <winsock2.h> is incompatible and if the
application wants to use the latter, it may be convenient to define
wxUSE_WINSOCK2 when building wxWidgets instead of having to work around
winsock.h implicit inclusion from include/wx/msw/wrapwin.h.

Closes https://github.com/wxWidgets/wxWidgets/pull/1122

build/cmake/setup.h.in

@@ -714,6 +714,8 @@
 
 #cmakedefine01 wxUSE_INICONF
 
+#cmakedefine01 wxUSE_WINSOCK2
+
 
 #cmakedefine01 wxUSE_DATEPICKCTRL_GENERIC