4 files changed, 49 insertions, 6 deletions
diff --git a/include/assert.h b/include/assert.h
index e5a963d..763b7b4 100644
--- a/include/assert.h
+++ b/include/assert.h
@@ -75,7 +75,6 @@
 #else
 # define assert_perror(errnum)  \
   ((void)((errnum == 0) ? 0 : (__assert_fail(NULL, errnum, __FILE__, __LINE__, __func__), 0)))
-__USE_GNU
 #endif
 
 
@@ -89,8 +88,8 @@ __USE_GNU
  * @param  line        The line in the source code whence the assertion was made
  * @param  func        The function in the source code whence the assertion was made
  */
-void __GCC_ONLY(__attribute__((noreturn, nonnull(3, 4, 5))))
-__assert_fail(const char* expression, int errnum, const char* file, int line, const char* func);
+void __assert_fail(const char* expression, int errnum, const char* file, int line, const char* func)
+  __noreturn __GCC_ONLY(__attribute__((nonnull(3, 4, 5))));
 
 
 
diff --git a/include/slibc/features.h b/include/slibc/features.h
index a755f71..0cbc326 100644
--- a/include/slibc/features.h
+++ b/include/slibc/features.h
@@ -19,6 +19,31 @@
 #define _SLIBC_FEATURES_H
 
 
+/* These definitions are only to be used in slibc header-files. */
+
+
+/**
+ * Is C11, or newer, used?
+ */
+#if __STDC_VERSION__ >= 201112L
+# define __C11__
+#endif
+
+/**
+ * Is C99, or newer, used?
+ */
+#if __STDC_VERSION__ >= 199901L
+# define __C99__
+#endif
+
+/**
+ * Is C90, or newer, used?
+ */
+#if defined(__STDC_VERSION__) || defined(__STDC__)
+# define __C90__
+#endif
+
+
 
 /**
  * Macro used to exclude code unless GCC is used.
@@ -30,6 +55,19 @@
 #endif
 
 
+/**
+ * Specifies that a function never returns, that is,
+ * the process exits before the function returns.
+ */
+#if !defined(__C11__) && defined(__GNUC__)
+# define __noreturn  __attribute__((noreturn))
+#elif defined(__C11__)
+# define __noreturn  _Noreturn
+#else
+# define __noreturn  /* ignore */
+#endif
+
+
 
 #endif
 
diff --git a/include/stdalign.h b/include/stdalign.h
index bce8f5d..b247b2c 100644
--- a/include/stdalign.h
+++ b/include/stdalign.h
@@ -20,13 +20,16 @@
 #include <slibc/version.h>
 
 
+#include <slibc/features.h>
+
+
 
 /**
  * Specify the alignment of a variable.
  * 
  * @param  type  The type whose alignment shall be used.
  */
-#if __STDC_VERSION__ < 201112L && defined(__GNUC__)
+#if !defined(__C11__) && defined(__GNUC__)
 # define _Alignas(type)  __attribute__((__aligned__(type)))
 #endif
 #define alignas(type)  _Alignas(type)
@@ -38,7 +41,7 @@
  * @param   type  The type.
  * @return        The alignment of the type.
  */
-#if __STDC_VERSION__ < 201112L && defined(__GNUC__)
+#if !defined(__C11__) && defined(__GNUC__)
 # define _Alignof(type)  __alignof__(type)
 #endif
 #define alignof(type)  _Alignof(type)
diff --git a/include/stdnoreturn.h b/include/stdnoreturn.h
index fd54d1b..19c6016 100644
--- a/include/stdnoreturn.h
+++ b/include/stdnoreturn.h
@@ -20,12 +20,15 @@
 #include <slibc/version.h>
 
 
+#include <slibc/features.h>
+
+
 
 /**
  * Specifies that a function never returns, that is,
  * the process exits before the function returns.
  */
-#if __STDC_VERSION__ < 201112L && defined(__GNUC__)
+#if !defined(__C11__) && defined(__GNUC__)
 # define _Noreturn  __attribute__((noreturn))
 #endif
 #define noreturn  _Noreturn