Use AC_USE_SYSTEM_EXTENSIONS

Defining _XOPEN_SOURCE explicitly in the source file leads to all kinds
of cross-platform headaches, plus incompatibilities between
configure-time capabilities and compile-time features. Let autoconf sort
out how to expose the largest number of functions in a way that is
consistent between configuration and compilation.

src/sql/mdbsql.c

@@ -17,9 +17,6 @@
  */
 
 #include <stdarg.h>
-#ifndef _XOPEN_SOURCE
-#define _XOPEN_SOURCE 700
-#endif
 #include "mdbsql.h"
 
 #ifdef HAVE_STRPTIME