add a conveniant way to detect memory leaks in Visual Studio

#ifdef'ed out for other built types.

MSVC's built-in memory leak detection is of limited capability due
to conflicts with placement new and missing stacktraces or even line
numbers.

This method uses the Visual Leak Detector http://www.vld.codeplex.com
LGPL'ed; I recommend setting StartDisabled = yes and
AggregateDuplicates = yes. This reduces a bit the noise caused by
static pointers initialized with new. There are a lot of leaks
reported for which I don't know whether they are some sort of false
positives.  I hope they vanish once that someone cleans up the
valgrind log. :)

src/game.cpp

@@ -43,6 +43,10 @@
 #include <boost/iostreams/copy.hpp>
 #include <boost/iostreams/filter/gzip.hpp>
 
+#ifdef HAVE_VISUAL_LEAK_DETECTOR
+#include "vld.h"
+#endif
+
 // Minimum stack cookie to prevent stack overflow on AmigaOS4
 #ifdef __amigaos4__
 const char __attribute__((used)) stackcookie[] = "\0$STACK: 16000000";
@@ -586,6 +590,11 @@ void init_custom_malloc();
 
 int main(int argc, char** argv)
 {
+
+#ifdef HAVE_VISUAL_LEAK_DETECTOR
+	VLDEnable();
+#endif
+
 #if defined(_OPENMP) && !defined(_WIN32) && !defined(__APPLE__)
 	// Wesnoth is a special case for OMP
 	// OMP wait strategy is to have threads busy-loop for 100ms