fix MSVC OpenGL linkage warnings & mingw linker problems. use SDL.lib also for MinGW.

BBGE/Core.cpp

@@ -1797,7 +1797,7 @@ void Core::setSDLGLAttributes()
 #define GLAPIENTRY
 #endif
 
-#if BBGE_BUILD_OPENGL_DYNAMIC
+#ifdef BBGE_BUILD_OPENGL_DYNAMIC
 #define GL_FUNC(ret,fn,params,call,rt) \
     extern "C" { \
         static ret (GLAPIENTRY *p##fn) params = NULL; \

BBGE/Effects.cpp

@@ -32,9 +32,7 @@ GLuint emptyTexture()											// Create An Empty Texture
 
 	// Create Storage Space For Texture Data (128x128x4)
 	data = (unsigned int*)new GLuint[((quality * quality)* 4 * sizeof(unsigned int))];
-#ifdef BBGE_BUILD_WINDOWS
-	ZeroMemory(data,((quality * quality)* 4 * sizeof(unsigned int)));	// Clear Storage Memory
-#endif
+	memset(data, 0, ((quality * quality)* 4 * sizeof(unsigned int)));	// Clear Storage Memory
 
 #ifdef BBGE_BUILD_OPENGL
 	glGenTextures(1, &txtnumber);								// Create 1 Texture

BBGE/GL/gl.h

@@ -35,9 +35,18 @@ extern "C" {
 #define APIENTRY
 #endif
 
-#ifndef WIN32
+// BBGE HACK:
+// Because Win32 uses __declspec(dllimport) for OpenGL function imports,
+// although Core.cpp defines its own functions using the same names, which are just accessing
+// OpenGL function pointers acquired elsewhere, this leads to inconsistent linkage between
+// declaration in gl.h and the actual function definition in Core.cpp.
+// So, we use this little hack to disable external linkage globally.
+#undef WINGDIAPI
+#define WINGDIAPI extern
+
+/*#ifndef WIN32
 #define WINGDIAPI
-#endif
+#endif*/
 
 #ifndef GLAPI
 # ifdef _WIN32