configure: Disable warning C4703 with MSVC

This disables warnings about potentially uninitialized local pointer
variables.  Disabling the warning is in line with what we do for gcc.

configure

@@ -3056,7 +3056,7 @@ msvc_flags(){
             -Wall)                echo -W4 -wd4244 -wd4127 -wd4018 -wd4389     \
                                        -wd4146 -wd4057 -wd4204 -wd4706 -wd4305 \
                                        -wd4152 -wd4324 -we4013 -wd4100 -wd4214 \
-                                       -wd4273 -wd4554 -wd4701 ;;
+                                       -wd4273 -wd4554 -wd4701 -wd4703 ;;
         esac
     done
 }