just use build-ins when C is modern enough
and other glibc defines that need enablement
diff -ru a/rc.h b/rc.h
--- a/rc.h	2025-01-05 21:16:39.487999355 +0400
+++ b/rc.h	2025-01-05 21:21:13.688559101 +0400
@@ -44,9 +44,14 @@
 	eError, eBreak, eReturn, eVarstack, eArena, eFifo, eFd
 } ecodes;
 
+#if __STDC_VERSION__ <= 201710L
 typedef enum bool {
 	FALSE, TRUE
 } bool;
+#else
+#define FALSE false
+#define TRUE true
+#endif
 
 typedef enum redirtype {
 	rFrom, rCreate, rAppend, rHeredoc, rHerestring
diff -ru a/configure.ac b/configure.ac
--- a/configure.ac	2025-01-05 21:33:03.952267739 +0400
+++ b/configure.ac	2025-01-05 21:33:20.702194221 +0400
@@ -1,5 +1,6 @@
 dnl Our package name, version, ...
 AC_INIT([rc], [1.7.4])
+AC_USE_SYSTEM_EXTENSIONS
 
 dnl ... and release date
 RELDATE=`date -I`