diff --git a/configure.ac b/configure.ac
index f579968..1136d0a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -399,14 +399,18 @@ public:
   [AC_MSG_RESULT([no])])
 AC_LANG_POP(C++)
 
-AC_CHECK_DECLS([__builtin_bswap64, __builtin_ctz])
-
 AC_CHECK_DECL([mach_absolute_time],
   [AC_DEFINE([HAVE_MACH_ABSOLUTE_TIME], [1],
      [Define if mach_absolute_time is available.])],
   , [[#include <mach/mach_time.h>]])
 
-AC_CHECK_DECLS([be64toh, betoh64], [], [],
+
+AC_CHECK_DECLS([__builtin_ctz])
+
+AC_CHECK_DECLS([ffs], [], [],
+  [[#include <strings.h>]])
+
+AC_CHECK_DECLS([be64toh, betoh64, bswap64, __builtin_bswap64], [], [],
   [[#if defined(HAVE_ENDIAN_H)
 #include <endian.h>
 #elif defined(HAVE_SYS_ENDIAN_H)
diff --git a/src/crypto/ocb.cc b/src/crypto/ocb.cc
index 1f73475..f125858 100644
--- a/src/crypto/ocb.cc
+++ b/src/crypto/ocb.cc
@@ -80,6 +80,15 @@
 #include "ae.h"
 #include <stdlib.h>
 #include <string.h>
+#if defined(HAVE_STRINGS_H)
+#include <strings.h>
+#endif
+#if defined(HAVE_ENDIAN_H)
+#include <endian.h>
+#elif defined(HAVE_SYS_ENDIAN_H)
+#include <sys/types.h>
+#include <sys/endian.h>
+#endif
 
 /* Define standard sized integers                                          */
 #if defined(_MSC_VER) && (_MSC_VER < 1600)
@@ -110,6 +119,8 @@
 
 #if _MSC_VER
 	#define bswap64(x) _byteswap_uint64(x)
+#elif HAVE_DECL_BSWAP64
+	/* nothing */
 #elif HAVE_DECL___BUILTIN_BSWAP64
 	#define bswap64(x) __builtin_bswap64(x)           /* GCC 4.3+ */
 #else
@@ -130,6 +141,8 @@
 	static inline unsigned ntz(unsigned x) {_BitScanForward(&x,x);return x;}
 #elif HAVE_DECL___BUILTIN_CTZ
 	#define ntz(x)     __builtin_ctz((unsigned)(x))   /* GCC 3.4+ */
+#elif HAVE_DECL_FFS
+	#define ntz(x)     (ffs(x) - 1)
 #else
 	#if (L_TABLE_SZ <= 9) && (L_TABLE_SZ_IS_ENOUGH)   /* < 2^13 byte texts */
 	static inline unsigned ntz(unsigned x) {