Require glib2 2.68 or later (for g_memdup2) (#400)

include/mdbfakeglib.h

@@ -117,6 +117,7 @@ typedef struct GOptionContext {
 #define g_malloc malloc
 #define g_free free
 #define g_realloc realloc
+#define g_memdup2 g_memdup
 
 #define	G_STR_DELIMITERS "_-|> <."
 

include/mdbprivate.h

@@ -26,10 +26,6 @@
  * used within mdbtools so they won't be exported to calling programs.
  */
 
-#ifndef HAVE_G_MEMDUP2
-#define g_memdup2 g_memdup
-#endif
-
 #ifdef __cplusplus
   extern "C" {
 #endif