Make --enable-snmp cross compile and make libcrypto optional with --without-crypto

Autoconfig work by me, the rest was done by
"Kirill K. Smirnov" <lich@math.spbu.ru>
diff --git a/zebra/zebra_snmp.c b/zebra/zebra_snmp.c
index 4536026..7e66e2f 100644
--- a/zebra/zebra_snmp.c
+++ b/zebra/zebra_snmp.c
@@ -24,10 +24,12 @@
 #ifdef HAVE_SNMP
 #ifdef HAVE_NETSNMP
 #include <net-snmp/net-snmp-config.h>
-#endif
+#include <net-snmp/net-snmp-includes.h>
+#else
 #include <asn1.h>
 #include <snmp.h>
 #include <snmp_impl.h>
+#endif
 
 #include "if.h"
 #include "log.h"