Avoid implicit declaration memset in have_fpos_pos.c probe

Otherwise, the probe result changes with compilers which do not
support implicit function declarations, a language feature that
was removed from C in 1999.

have_fpos_pos.c

@@ -28,6 +28,10 @@
 #include <stdio.h>
 #include "have_fgetsetpos.h"
 #include "have_posscl.h"
+#include "have_string.h"
+#ifdef HAVE_STRING_H
+# include <string.h>
+#endif
 
 
 #include "banned.h"	/* include after system header <> includes */