Remove unneeded autoconf macro AC_HEADER_STDBOOL

strace code assumes C99.

* configure.ac (AC_HEADER_STDBOOL): Remove.
* defs.h: Include <stdbool.h> unconditionally.
diff --git a/defs.h b/defs.h
index 444ffeb..07de67a 100644
--- a/defs.h
+++ b/defs.h
@@ -35,9 +35,7 @@
 #endif
 
 #include <features.h>
-#ifdef HAVE_STDBOOL_H
-# include <stdbool.h>
-#endif
+#include <stdbool.h>
 #include <stdint.h>
 #include <inttypes.h>
 #include <sys/types.h>