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/state.c

@@ -4,8 +4,8 @@
  * State machine (singleton) for nvmutil data.
  */
 
-#ifndef _POSIX_C_SOURCE
-#define _POSIX_C_SOURCE 200809L
+#ifndef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 700
 #endif
 
 #include <sys/types.h>