Raise _XOPEN_SOURCE from 500 to 700

In OpenBSD, _XOPEN_SOURCE 500 forces _POSIX_C_SOURCE down to 199506,
which hides the declarations of openat() and fdopendir() and causes
compiler warnings.

Don't set _POSIX_C_SOURCE, because _XOPEN_SOURCE may set
_POSIX_C_SOURCE to a different value.

plat/cpm/emu/fileio.c

@@ -1,5 +1,4 @@
-#define _XOPEN_SOURCE 500
-#define _POSIX_C_SOURCE 200809
+#define _XOPEN_SOURCE 700
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>