diff --git a/util/libreboot-utils/include/common.h b/util/libreboot-utils/include/common.h
index 21be1898..a7099f35 100644
--- a/util/libreboot-utils/include/common.h
+++ b/util/libreboot-utils/include/common.h
@@ -13,18 +13,6 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <limits.h>
-#include <errno.h>
-
-/* for linux getrandom
- */
-#if defined(__linux__)
-#include <sys/random.h>
-#include <sys/syscall.h>
-#endif
-
-#ifdef __OpenBSD__ /* for pledge */
-#include <unistd.h>
-#endif
 
 /* dangerously cool macros:
  */
@@ -49,10 +37,6 @@
 /* system prototypes
  */
 
-int fchmod(int fd, mode_t mode);
-char *realpath(const char *path,
-    char *resolved);
-
 #ifndef USE_URANDOM
 #define USE_URANDOM 0
 #endif
diff --git a/util/libreboot-utils/lib/file.c b/util/libreboot-utils/lib/file.c
index 685cdf9a..3f15aabf 100644
--- a/util/libreboot-utils/lib/file.c
+++ b/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
diff --git a/util/libreboot-utils/lib/mkhtemp.c b/util/libreboot-utils/lib/mkhtemp.c
index 46f6419f..11a5bdb9 100644
--- a/util/libreboot-utils/lib/mkhtemp.c
+++ b/util/libreboot-utils/lib/mkhtemp.c
@@ -4,8 +4,23 @@
  * Hardened mktemp (be nice to the demon).
  */
 
-#if !defined(_POSIX_C_SOURCE)
-#define _POSIX_C_SOURCE 200809L
+#ifndef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 700
+#endif
+
+/* for openat2 / fast path: */
+#ifdef __linux__
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE 1
+#endif
+#include <sys/syscall.h>
+#include <linux/openat2.h>
+#ifndef O_TMPFILE
+#define O_TMPFILE 020000000
+#endif
+#ifndef AT_EMPTY_PATH
+#define AT_EMPTY_PATH 0x1000
+#endif
 #endif
 
 #include <sys/types.h>
@@ -18,18 +33,6 @@
 #include <string.h>
 #include <unistd.h>
 
-/* for openat2 / fast path: */
-#ifdef __linux__
-#include <linux/openat2.h>
-#include <sys/syscall.h>
-#ifndef O_TMPFILE
-#define O_TMPFILE 020000000
-#endif
-#ifndef AT_EMPTY_PATH
-#define AT_EMPTY_PATH 0x1000
-#endif
-#endif
-
 #include "../include/common.h"
 
 /* note: tmpdir is an override of TMPDIR or /tmp or /var/tmp */
diff --git a/util/libreboot-utils/lib/rand.c b/util/libreboot-utils/lib/rand.c
index 8c3fbf4c..726d5cd2 100644
--- a/util/libreboot-utils/lib/rand.c
+++ b/util/libreboot-utils/lib/rand.c
@@ -7,19 +7,21 @@
 #ifndef RAND_H
 #define RAND_H
 
-#ifdef __OpenBSD__
-#include <sys/param.h>
-#endif
-#include <sys/types.h>
-
 #if defined(USE_URANDOM) && \
     ((USE_URANDOM) > 0)
 #include <fcntl.h> /* if not arc4random: /dev/urandom */
 #elif defined(__linux__)
-#include <sys/random.h>
-#include <sys/syscall.h>
-long syscall(long number, ...);
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE 1
 #endif
+#include <sys/syscall.h>
+#include <sys/random.h>
+#endif
+
+#ifdef __OpenBSD__
+#include <sys/param.h>
+#endif
+#include <sys/types.h>
 
 #include <errno.h>
 #include <fcntl.h>
diff --git a/util/libreboot-utils/lib/state.c b/util/libreboot-utils/lib/state.c
index a7c85bfb..78e15134 100644
--- a/util/libreboot-utils/lib/state.c
+++ b/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>
diff --git a/util/libreboot-utils/lottery.c b/util/libreboot-utils/lottery.c
index a494eff1..1648cbc7 100644
--- a/util/libreboot-utils/lottery.c
+++ b/util/libreboot-utils/lottery.c
@@ -2,11 +2,6 @@
  * Copyright (c) 2026 Leah Rowe <leah@libreboot.org>                    /| |\
    Something something non-determinism                                   / \ */
 
-#if !defined(_POSIX_C_SOURCE)
-/* for openat2 on linux */
-#define _POSIX_C_SOURCE 200809L
-#endif
-
 #include <ctype.h>
 #include <stddef.h>
 #include <stdio.h>
diff --git a/util/libreboot-utils/mkhtemp.c b/util/libreboot-utils/mkhtemp.c
index c8ddb1b1..86aab536 100644
--- a/util/libreboot-utils/mkhtemp.c
+++ b/util/libreboot-utils/mkhtemp.c
@@ -16,9 +16,8 @@
  * while the specification that it implements evolves.
  */
 
-#if !defined(_POSIX_C_SOURCE)
-/* for openat2 on linux */
-#define _POSIX_C_SOURCE 200809L
+#ifndef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 700
 #endif
 
 #include <sys/types.h>
diff --git a/util/libreboot-utils/nvmutil.c b/util/libreboot-utils/nvmutil.c
index c805f523..ee20e851 100644
--- a/util/libreboot-utils/nvmutil.c
+++ b/util/libreboot-utils/nvmutil.c
@@ -6,10 +6,6 @@
  * These images configure your Intel Gigabit Ethernet adapter.
  */
 
-#ifndef _POSIX_C_SOURCE
-#define _POSIX_C_SOURCE 200809L
-#endif
-
 #include <sys/types.h>
 #include <sys/stat.h>