--- a/src/libs/libgroff/getopt.c
+++ b/src/libs/libgroff/getopt.c
@@ -122,7 +122,7 @@
    whose names are inconsistent.  */
 
 #ifndef getenv
-extern char *getenv ();
+extern char *getenv (const char *);
 #endif
 
 #endif /* not __GNU_LIBRARY__ */