Fix build with -fno-common

Index: twhich.c
--- twhich.c.orig
+++ twhich.c
@@ -23,6 +23,7 @@
 const EVP_MD    *md;
 
 int		case_sensitive = 1;
+int		cksum;
 int		tran_format = -1; 
 
 /*
