diff --git a/include/arpa/inet.h b/include/arpa/inet.h
index 82e2ac1bf4afaf149e77924663f0427b12044b76..9293840399b8e84fc8c34db12d120556fd279c25 100644
--- a/include/arpa/inet.h
+++ b/include/arpa/inet.h
@@ -5,12 +5,7 @@
 extern "C" {
 #endif
 
-#if __STDC_VERSION__ >= 199901L
-#define __restrict restrict
-#elif !defined(__GNUC__)
-#define __restrict
-#endif
-
+#include <features.h>
 #include <netinet/in.h>
 #include <inttypes.h>
 
diff --git a/include/byteswap.h b/include/byteswap.h
index bf222d3cb249517a31dfedbaa10a3f9266ea35c6..00b9df3c9fa2b1511105ef60efca7650607a9f52 100644
--- a/include/byteswap.h
+++ b/include/byteswap.h
@@ -1,12 +1,9 @@
 #ifndef _BYTESWAP_H
 #define _BYTESWAP_H
 
+#include <features.h>
 #include <stdint.h>
 
-#if __STDC_VERSION__ >= 199901L || defined(__cplusplus)
-#define __inline inline
-#endif
-
 static __inline uint16_t __bswap_16(uint16_t __x)
 {
 	return __x<<8 | __x>>8;
diff --git a/include/iconv.h b/include/iconv.h
index cef06f604953fede48996271e60f3db89de9f8d7..ebe9bfda3926a6a532b4aefc1db8e06a16ec0cdf 100644
--- a/include/iconv.h
+++ b/include/iconv.h
@@ -5,11 +5,7 @@
 extern "C" {
 #endif
 
-#if __STDC_VERSION__ >= 199901L
-#define __restrict restrict
-#elif !defined(__GNUC__)
-#define __restrict
-#endif
+#include <features.h>
 
 #define __NEED_size_t
 
diff --git a/include/inttypes.h b/include/inttypes.h
index 23acc5bec142e2acc041f654e2ec434a144efd31..3f0339ce340e3382d69d5a7f32117d6808e89fa1 100644
--- a/include/inttypes.h
+++ b/include/inttypes.h
@@ -5,17 +5,12 @@
 extern "C" {
 #endif
 
+#include <features.h>
 #include <stdint.h>
 
 #define __NEED_wchar_t
 #include <bits/alltypes.h>
 
-#if __STDC_VERSION__ >= 199901L
-#define __restrict restrict
-#elif !defined(__GNUC__)
-#define __restrict
-#endif
-
 typedef struct { intmax_t quot, rem; } imaxdiv_t;
 
 intmax_t imaxabs(intmax_t);
diff --git a/include/monetary.h b/include/monetary.h
index 11dabb2341f4687424ba10ed50eff2eddf078fe8..a91fa565574758726977376659b9890a2380087d 100644
--- a/include/monetary.h
+++ b/include/monetary.h
@@ -5,11 +5,7 @@
 extern "C" {
 #endif
 
-#if __STDC_VERSION__ >= 199901L
-#define __restrict restrict
-#elif !defined(__GNUC__)
-#define __restrict
-#endif
+#include <features.h>
 
 #define __NEED_ssize_t
 #define __NEED_size_t
diff --git a/include/mqueue.h b/include/mqueue.h
index bd35842c42ca7f5b42e11b1625dbb5ea3c33601e..f5cbe79656b3ab2fdcfc18c91229649e77a2c82e 100644
--- a/include/mqueue.h
+++ b/include/mqueue.h
@@ -4,11 +4,7 @@
 extern "C" {
 #endif
 
-#if __STDC_VERSION__ >= 199901L
-#define __restrict restrict
-#elif !defined(__GNUC__)
-#define __restrict
-#endif
+#include <features.h>
 
 #define __NEED_size_t
 #define __NEED_ssize_t
diff --git a/include/pthread.h b/include/pthread.h
index bc0d9f1c32bc3ef8a8ca6ea409eb4d5e0462434a..74d86006a442e1f577358ac390e2ed5cdffbf0f0 100644
--- a/include/pthread.h
+++ b/include/pthread.h
@@ -4,18 +4,7 @@
 extern "C" {
 #endif
 
-#if __STDC_VERSION__ >= 199901L
-#define __restrict restrict
-#elif !defined(__GNUC__)
-#define __restrict
-#endif
-
-#if __STDC_VERSION__ >= 201112L
-#elif defined(__GNUC__)
-#define _Noreturn __attribute__((__noreturn__))
-#else
-#define _Noreturn
-#endif
+#include <features.h>
 
 #define __NEED_time_t
 #define __NEED_clockid_t
diff --git a/include/regex.h b/include/regex.h
index b7167b876e1a689bd3faaeb12aedffb534d4d25c..2eac1ebf9bdd6a02c97c5360724561a5695573c2 100644
--- a/include/regex.h
+++ b/include/regex.h
@@ -5,11 +5,7 @@
 extern "C" {
 #endif
 
-#if __STDC_VERSION__ >= 199901L
-#define __restrict restrict
-#elif !defined(__GNUC__)
-#define __restrict
-#endif
+#include <features.h>
 
 #define __NEED_size_t
 
diff --git a/include/semaphore.h b/include/semaphore.h
index 724957fbe44635b2e080149908fec5035f7c8e5b..20d46f0de2a70c6f4c1c9f10891af2e7b5dec787 100644
--- a/include/semaphore.h
+++ b/include/semaphore.h
@@ -4,11 +4,7 @@
 extern "C" {
 #endif
 
-#if __STDC_VERSION__ >= 199901L
-#define __restrict restrict
-#elif !defined(__GNUC__)
-#define __restrict
-#endif
+#include <features.h>
 
 #define __NEED_time_t
 #define __NEED_struct_timespec
diff --git a/include/spawn.h b/include/spawn.h
index c934d4238851da5345d76d912d5900ae345259ec..a28ae69168b4c54d6bc5972988a51347381ca954 100644
--- a/include/spawn.h
+++ b/include/spawn.h
@@ -5,11 +5,7 @@
 extern "C" {
 #endif
 
-#if __STDC_VERSION__ >= 199901L
-#define __restrict restrict
-#elif !defined(__GNUC__)
-#define __restrict
-#endif
+#include <features.h>
 
 #define __NEED_mode_t
 #define __NEED_pid_t
diff --git a/include/sys/select.h b/include/sys/select.h
index dd4176dd2980c4957450feb00c3d7d92761fb2d7..c5a2877f87ac604c0082c4dbb0c4dc4442bd4b3d 100644
--- a/include/sys/select.h
+++ b/include/sys/select.h
@@ -4,11 +4,7 @@
 extern "C" {
 #endif
 
-#if __STDC_VERSION__ >= 199901L
-#define __restrict restrict
-#elif !defined(__GNUC__)
-#define __restrict
-#endif
+#include <features.h>
 
 #define __NEED_size_t
 #define __NEED_time_t
diff --git a/include/sys/socket.h b/include/sys/socket.h
index 7024d232df0928ec651a1c54f3a002bf32b90c87..97d78871ae1413f730c1f1d449901a2e166850e7 100644
--- a/include/sys/socket.h
+++ b/include/sys/socket.h
@@ -4,11 +4,7 @@
 extern "C" {
 #endif
 
-#if __STDC_VERSION__ >= 199901L
-#define __restrict restrict
-#elif !defined(__GNUC__)
-#define __restrict
-#endif
+#include <features.h>
 
 #define __NEED_socklen_t
 #define __NEED_sa_family_t
diff --git a/include/sys/syscall.h b/include/sys/syscall.h
index 154e5ab5636f3e8060df2b8dda6fbbd69c6b75e1..62564d7d2e10273a5e7e4f484e2b0d741a9ea6c9 100644
--- a/include/sys/syscall.h
+++ b/include/sys/syscall.h
@@ -4,9 +4,7 @@
 extern "C" {
 #endif
 
-#if __STDC_VERSION__ >= 199901L || defined(__cplusplus)
-#define __inline inline
-#endif
+#include <features.h>
 
 long __syscall_ret(unsigned long);
 long __syscall(long, ...);
diff --git a/include/wordexp.h b/include/wordexp.h
index e7eb3255ded8ffede3b8a24c153e5c875a9c65be..d12081e80130ca235315fed808fe2d267b7c723c 100644
--- a/include/wordexp.h
+++ b/include/wordexp.h
@@ -5,11 +5,7 @@
 extern "C" {
 #endif
 
-#if __STDC_VERSION__ >= 199901L
-#define __restrict restrict
-#elif !defined(__GNUC__)
-#define __restrict
-#endif
+#include <features.h>
 
 #define __NEED_size_t