Changed test for -u switch in order to enable other wmake switches.

The var %MAKEFLAGS is only set in 3 cases: if set as environment
var or as macro definition from commandline, and either with the
-u or -ms switch. Since all these cases are unlikely for the average
user it should be safe to only test if %MAKEFLAGS is defined; this
has the benefit that now all other switches can be used again in
addition to the -u which was formerly not possible.