make openssl optional

https://bugs.gentoo.org/335436

--- ping6.c
+++ ping6.c
@@ -161,7 +161,9 @@
 
 static int icmp_sock;
 
+#ifdef HAVE_OPENSSL
 #include <openssl/md5.h>
+#endif
 
 /* Node Information query */
 int ni_query = -1;
@@ -478,6 +480,7 @@
 
 char *ni_groupaddr(const char *name)
 {
+#ifdef HAVE_OPENSSL
 	MD5_CTX ctxt;
 	__u8 digest[16];
 	static char nigroup_buf[INET6_ADDRSTRLEN + 1 + IFNAMSIZ];
@@ -518,6 +521,10 @@
 	if (q)
 		strcat(nigroup_buf, q);
 	return nigroup_buf;
+#else
+	fprintf(stderr, "ping6: function not available; openssl disabled\n");
+	exit(1);
+#endif
 }
 
 int main(int argc, char *argv[])