Re-enable fast concatenation on macOS again

The old compilers that had issues with it should now long be unsupported

Change-Id: I49a30d1533814877ae4fde9c9bd4e3e64528dfa3
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

qtcreator.pri

@@ -201,8 +201,9 @@ DEFINES += \
     QT_CREATOR \
     QT_NO_CAST_TO_ASCII \
     QT_RESTRICTED_CAST_FROM_ASCII \
-    QT_DISABLE_DEPRECATED_BEFORE=0x050600
-!macx:DEFINES += QT_USE_FAST_OPERATOR_PLUS QT_USE_FAST_CONCATENATION
+    QT_DISABLE_DEPRECATED_BEFORE=0x050600 \
+    QT_USE_FAST_OPERATOR_PLUS \
+    QT_USE_FAST_CONCATENATION
 
 unix {
     CONFIG(debug, debug|release):OBJECTS_DIR = $${OUT_PWD}/.obj/debug-shared