Let the child bjam commands inherit stdin, as some commands assume it's available. (fixes #1577)

[SVN r49883]

historic/jam/src/execnt.c

@@ -403,6 +403,9 @@ void exec_cmd
             si.hStdError = cmdtab[ slot ].pipe_out[ 1 ];
         }
 
+        /* Let the child inherit stdin, as some commands assume it's available. */
+        si.hStdInput = GetStdHandle(STD_INPUT_HANDLE);
+
         /* Save the operation for exec_wait() to find. */
         cmdtab[ slot ].func = func;
         cmdtab[ slot ].closure = closure;