[Go] Properly set _cgo_* functions on Windows

Based on the work of Wei Guangjing[1].
On Windows, dynamic binding of methods is not possible. So we use DllMain to do
that.
[1] https://groups.google.com/forum/#!topic/golang-nuts/9L0U4Q7AtyE
[2] swig#100

Closes swig#100

Signed-off-by: Steeve Morin <steeve.morin@gmail.com>

Lib/go/goruntime.swg

@@ -56,14 +56,39 @@ swiggo_size_assert(double, 8)
 #ifdef __cplusplus
 extern "C" {
 #endif
+#ifdef defined(_WIN32) || defined(__WIN32__) || defined(__CYGWIN__)
+void (*crosscall2)(void (*fn)(void *, int), void *, int);
+char* (*_cgo_topofstack)(void) __attribute__ ((weak));
+void (*_cgo_allocate)(void *, int);
+void (*_cgo_panic)(void *, int);
+#else // defined(_WIN32) || defined(__WIN32__) || defined(__CYGWIN__)
 extern void crosscall2(void (*fn)(void *, int), void *, int);
 extern char* _cgo_topofstack(void) __attribute__ ((weak));
 extern void _cgo_allocate(void *, int);
 extern void _cgo_panic(void *, int);
+#endif // defined(_WIN32) || defined(__WIN32__) || defined(__CYGWIN__)
 #ifdef __cplusplus
 }
 #endif
 
+#ifdef defined(_WIN32) || defined(__WIN32__) || defined(__CYGWIN__)
+extern "C" BOOL WINAPI DllMain (HANDLE hDll, DWORD dwReason, LPVOID lpReserved)
+{
+    switch (dwReason)
+    {
+        case DLL_PROCESS_ATTACH:
+            HMODULE hModule;
+            hModule = GetModuleHandle(NULL);
+            crosscall2 = (void (*)(void (*fn)(void *, int), void *, int))GetProcAddress(hModule, "crosscall2");
+            _cgo_topofstack = (char* (*)(void))GetProcAddress(hModule, "_cgo_topofstack");
+            _cgo_allocate = (void (*)(void *, int))GetProcAddress(hModule, "_cgo_allocate");
+            _cgo_panic = (void (*)(void *, int))GetProcAddress(hModule, "_cgo_panic");
+            break;
+    }
+    return TRUE;
+}
+#endif
+
 static char *_swig_topofstack() {
   if (_cgo_topofstack) {
     return _cgo_topofstack();