add requires() function and REQUIRES magic variable

follow suit with qmake ...

Change-Id: I6afc3898a01cc9926b9652b7f0f66becc255f01b
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>

src/shared/proparser/qmakebuiltins.cpp

@@ -1023,8 +1023,10 @@ QMakeEvaluator::VisitReturn QMakeEvaluator::evaluateConditionalFunction(
             }
         }
         return ReturnFalse;
-#if 0
+#ifdef PROEVALUATOR_FULL
     case T_REQUIRES:
+        checkRequirements(args);
+        return ReturnFalse; // Another qmake breakage
 #endif
     case T_EVAL: {
             VisitReturn ret = ReturnFalse;