Add build option for using Visual Leak Detector for debugging memory leaks. Initial report:

- IfcConvert (no arguments): 2 leaks (140 bytes) (from OCE)
- IfcConvert (acad2010_objects.ifc): 47 memory leaks (6,158 bytes)
- IfcGeomServer: 2 leaks (from OCE)
- IfcOpenHouse: 11,952 leaks (1,169,658 bytes)!
- IfcParseExamples (acad2010_objects.ifc): 40 memory leaks (4,564 bytes)

src/ifcconvert/IfcConvert.cpp

@@ -46,6 +46,10 @@
 #include <IGESControl_Controller.hxx>
 #include <Standard_Version.hxx>
 
+#if USE_VLC
+#include <vld.h>
+#endif
+
 static std::string DEFAULT_EXTENSION = "obj";
 
 void printVersion() {