qmake4Hook: enable parallel building with qmake4Hook by default

pkgs/development/libraries/qt-4.x/4.8/qmake-hook.sh

···
 
     $QMAKE PREFIX=$out $qmakeFlags
 
+    if ! [[ -v enableParallelBuilding ]]; then
+        enableParallelBuilding=1
+        echo "qmake4Hook: enabled parallel building"
+    fi
+
     runHook postConfigure
 }