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/examples/IfcParseExamples.cpp

@@ -19,6 +19,10 @@
 
 #include "../ifcparse/IfcFile.h"
 
+#if USE_VLC
+#include <vld.h>
+#endif
+
 using namespace IfcSchema;
 
 int main(int argc, char** argv) {