Add a warning if wspiapi.h doesn't exist.

ConfigureChecks.cmake

@@ -25,6 +25,9 @@ check_include_file(pty.h HAVE_PTY_H)
 check_include_file(terminos.h HAVE_TERMIOS_H)
 if (WIN32)
   check_include_file(wspiapi.h HAVE_WSPIAPI_H)
+  if (NOT HAVE_WSPIAPI_H)
+    message(STATUS "WARNING: Without wspiapi.h, this build will only work on Windows XP and newer versions")
+  endif (NOT HAVE_WSPIAPI_H)
   set(HAVE_GETADDRINFO TRUE)
   set(HAVE_GETHOSTBYNAME TRUE)
   set(HAVE_SELECT TRUE)