https://bugs.gentoo.org/711902

--- intercal-0.31-orig/src/perpet.c
+++ intercal-0.31/src/perpet.c
@@ -85,7 +85,7 @@
 /* function created by yacc */
 extern int yyparse(void);
 
-int yydebug;
+extern int yydebug;
 
 /* compilation options */
 bool compile_only; 	/* just compile into C, don't run the linker */