Double-wrap bool in case it's a keyword, not macro, for C23.
As typedef enum bool was guarded before and downstream user
tuxmath package takes care to use bool type only internally,
without exposing it over the network or to filesystem,
I act under presumption that this is ok to do in this
installable header.
https://bugs.gentoo.org/943953
--- a/src/t4k_common.h
+++ b/src/t4k_common.h
@@ -55,6 +55,7 @@

 #include <sys/types.h>
 #include <sys/stat.h>
+#include <stdbool.h>
 #include <fcntl.h>
 #include <unistd.h>
 #include <dirent.h>
@@ -124,6 +125,7 @@
 // #define _(String) gettext (String)

 #ifndef bool
+#if __STDC_VERSION__ <= 201710L
 typedef enum
 {
    false,
@@ -131,6 +133,7 @@
 }
 bool;
+#endif
 #endif

 // Debug macros
 #define DEBUGCODE(mask) if((mask) & debug_status)