bugfix [#24518] Blender wont compile with -Wall -Werror and COLLADA support

fix included in report from Martijn Berger (mberger)
made some small changes.

- use ints rather then unsigned long for printing, values are not likely to be very large.
- CMake remove strict flags from collada build dir since I had warnings in the collada headers.
- added xml2 to collada libraries else I couldnt get collada building.

source/blender/windowmanager/intern/wm_operators.c

@@ -1920,7 +1920,7 @@ static void WM_OT_save_mainfile(wmOperatorType *ot)
 
 #include "../../collada/collada.h"
 
-static int wm_collada_export_invoke(bContext *C, wmOperator *op, wmEvent *event)
+static int wm_collada_export_invoke(bContext *C, wmOperator *op, wmEvent *UNUSED(event))
 {	
 	if(!RNA_property_is_set(op->ptr, "filepath")) {
 		char filepath[FILE_MAX];