build: Add check that a value was specified in SetConfigVar.

e.g. if one merely has "SetConfigVar DEBUG : HAIKU_TOP src system kernel ;"
this won't actually set a value for DEBUG, and thus it won't use one
of the pre-set debug objects directories, and so all the objects will
wind up in the root of wherever jam is run.

build/jam/ConfigRules

@@ -42,6 +42,10 @@ rule SetConfigVar
 	local config = [ ConfigObject $(2) ] ;
 	local scope = $(4:E=global) ;
 
+	if ! $(3) {
+		Exit "Error: no value specified for ConfigVar '$(var)'!" ;
+	}
+
 	$(var) on $(config) = $(3) ;
 	__set_$(var) on $(config) = $(scope) ;