mdbx-windows: add mdbx_strdup() to avoid dependency from CRT.

12 of 17 for https://github.com/leo-yuriev/libmdbx/issues/43

Change-Id: Ib1379d75dc25e34f56daf8988848c41f59b6cd6b

src/osal.c

@@ -288,6 +288,18 @@ void mdbx_memalign_free(void *ptr) {
 }
 #endif /* mdbx_memalign_free */
 
+#ifndef mdbx_strdup
+char *mdbx_strdup(const char *str) {
+  if (!str)
+    return NULL;
+  size_t bytes = strlen(str) + 1;
+  char *dup = mdbx_malloc(bytes);
+  if (dup)
+    memcpy(dup, str, bytes);
+  return dup;
+}
+#endif /* mdbx_strdup */
+
 /*----------------------------------------------------------------------------*/
 
 int mdbx_condmutex_init(mdbx_condmutex_t *condmutex) {