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/ifcgeomserver/IfcGeomServer.cpp

@@ -37,6 +37,10 @@
 
 #include "../ifcgeom/IfcGeomIterator.h"
 
+#if USE_VLC
+#include <vld.h>
+#endif
+
 using namespace boost;
 
 template <typename T>
@@ -282,7 +286,13 @@ public:
 	Bye() : Command(BYE) {};
 };
 
-int main () {
+int main ()
+{
+	// Memory leak debugging in MSVC debug mode
+#if defined(_MSC_VER) && defined(_DEBUG)
+	_CrtSetDbgFlag(_CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF);
+#endif
+
 	// Redirect stdout to this stream, so that involuntary 
 	// writes to stdout do not interfere with our protocol.
 	std::ostringstream oss;