Build by default with optimization level -O2 for GCC

before -O0 was used by default, that caused pretty bad performance.

Amd64, i6700k, 4200MHz, GNU/Linux (non-RT kernel), gcc 7.2.0

-------------------------------------
Optimization | EN/ENO |no EN/ENO |
-------------------------------------
default | 11 | 9.5 |
-O3 | 3.9 | 5.2 |
-O2 | 4 | 4.8 |
-Os | 4.1 | 3.5 |
-Ofast | 3.9 | 5.2 |
-------------------------------------

ARM, BBB Cortex-A8, 600Mhz, GNU/Linux, gcc 4.6.3

-------------------------------------
Optimization | EN/ENO |no EN/ENO |
-------------------------------------
default | 273 | 226 |
-O3 | 141.8 | 106.2 |
-O2 | 142 | 107 |
-Os | 152.5 | 112.2 |
-Ofast | 141.7 | 106.2 |
-------------------------------------

For embedded systems with size constaints (like Cortex-Mx, AVR and so
on) I usually use -Os. It gets pretty good results. For
GNU/Linux-based systems -O2 is usually a good choice, as you see the
test results.

// import Nevow.Athena

function init() {
  Nevow.Athena.Widget.fromAthenaID(1).callRemote('HMIexec', 'HMIinitialisation');
}

WebInterface.PLC = Nevow.Athena.Widget.subclass('WebInterface.PLC');
WebInterface.PLC.method(
	 'updateHMI',
	 function (self, data) {
	   d = self.callRemote('getPLCElement');
	   d.addCallback(
			 function liveElementReceived(le) {
				d2 = self.addChildWidgetFromWidgetInfo(le);
				d2.addCallback(
						function childAdded(widget) {
						var node = self.nodeById('content');
						node.replaceChild(widget.node, node.getElementsByTagName('div')[0]);
						init();
						});
				});
	   });

Divmod.Base.addLoadEvent(init);