Fix noreturn for old compilers.

FIXES

@@ -25,6 +25,10 @@ THIS SOFTWARE.
 This file lists all bug fixes, changes, etc., made since the AWK book
 was sent to the printers in August, 1987.
 
+April 16, 2020:
+	Handle old compilers that don't support C11 (for noreturn).
+	Thanks to Arnold Robbins.
+
 April 5, 2020:
 	Use <stdnoreturn.h> and noreturn instead of GCC attributes.
 	Thanks to GitHub user awkfan77. Closes PR #77.

awk.h

@@ -25,7 +25,11 @@ THIS SOFTWARE.
 #include <assert.h>
 #include <stdint.h>
 #include <stdbool.h>
+#if __STDC__ <= 199901L
+#define noreturn
+#else
 #include <stdnoreturn.h>
+#endif
 
 typedef double	Awkfloat;
 

main.c

@@ -22,7 +22,7 @@ ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF
 THIS SOFTWARE.
 ****************************************************************/
 
-const char	*version = "version 20200405";
+const char	*version = "version 20200416";
 
 #define DEBUG
 #include <stdio.h>