Taken from Gentoo. Fixes:

bashline.c:65:10: fatal error: builtins/builtext.h: No such file or directory
   65 | #include "builtins/builtext.h"  /* for read_builtin */
      |          ^~~~~~~~~~~~~~~~~~~~~

--- bash-5.1/Makefile.in
+++ bash-5.1/Makefile.in
@@ -584,6 +584,8 @@
 	ls -l $(Program)
 	-$(SIZE) $(Program)
 
+$(CSOURCES): $(DEFDIR)/builtext.h
+
 .build:	$(SOURCES) config.h Makefile version.h $(VERSPROG)
 	@echo
 	@echo "	  ***********************************************************"