lbutils: only use GNU SOURCE for syscall

and remove manual prototypes; fchmod, realpath
and so on rely on the _XOPEN_SOURCE macro.

the POSIX macro wasn't needed: _XOPEN_SOURCE
is sufficient.

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/libreboot-utils/lib/file.c

@@ -16,8 +16,17 @@ more correct usage example:
 long max = pathconf("/", _PC_PATH_MAX);
  */
 
-#ifndef _POSIX_C_SOURCE
-#define _POSIX_C_SOURCE 200809L
+#ifndef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 700
+#endif
+
+/* for openat2: */
+#ifdef __linux__
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE 1
+#endif
+#include <linux/openat2.h>
+#include <sys/syscall.h>
 #endif
 
 #include <sys/types.h>
@@ -30,13 +39,6 @@ long max = pathconf("/", _PC_PATH_MAX);
 #include <string.h>
 #include <unistd.h>
 
-/* for openat2: */
-#ifdef __linux__
-#include <linux/openat2.h>
-#include <sys/syscall.h>
-long syscall(long number, ...);
-#endif
-
 #include "../include/common.h"
 
 /* check that a file changed