guard _XOPEN_SOURCE defines against redefinition

Debian's ncurses pkg-config Cflags inject -D_XOPEN_SOURCE=600 on the
command line, which clashed with the hardcoded `#define _XOPEN_SOURCE 500`
in file.c and poolworker.c and failed the STRICT (-Werror) build with
-Wmacro-redefined. Wrap the defines in #ifndef so a stronger command-line
definition wins.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: mx4

lib/file/file.c

@@ -1,8 +1,12 @@
 #ifdef linux
+#ifndef _XOPEN_SOURCE
 #define _XOPEN_SOURCE 500
+#endif
 // for O_DIRECT
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
 #endif
+#endif
 
 #include <stdlib.h>
 #include <stdio.h>

lib/poolworker/poolworker.c

@@ -1,4 +1,6 @@
+#ifndef _XOPEN_SOURCE
 #define _XOPEN_SOURCE 500
+#endif
 #include <stdlib.h>
 #include <stdio.h>
 #include <pthread.h>