portable_endian.h: A more uniform fix for the various BSD flavours

See pull requests #59 and #83.

It looks like various BSD derivates have adopted the more common naming
conventions for the endian conversion macros, which means that we would
attempt to redefine them without need (potentially even to 'historic'
names that no longer exist).

Try to avoid this by properly checking for existing functions first.
Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>

include/portable_endian.h

@@ -41,22 +41,29 @@
 #	define __LITTLE_ENDIAN LITTLE_ENDIAN
 #	define __PDP_ENDIAN    PDP_ENDIAN
 
-#elif defined(__OpenBSD__) || defined(__FreeBSD__)
-
-#	include <sys/endian.h>
-
-#elif defined(__NetBSD__) || defined(__DragonFly__)
+#elif defined(__OpenBSD__) ||  defined(__NetBSD__) || defined(__FreeBSD__) || defined(__DragonFly__)
 
 #	include <sys/endian.h>
 
+/* For functions still missing, try to substitute 'historic' OpenBSD names */
+#ifndef be16toh
 #	define be16toh(x) betoh16(x)
+#endif
+#ifndef le16toh
 #	define le16toh(x) letoh16(x)
-
+#endif
+#ifndef be32toh
 #	define be32toh(x) betoh32(x)
+#endif
+#ifndef le32toh
 #	define le32toh(x) letoh32(x)
-
+#endif
+#ifndef be64toh
 #	define be64toh(x) betoh64(x)
+#endif
+#ifndef le64toh
 #	define le64toh(x) letoh64(x)
+#endif
 
 #elif defined(__WINDOWS__)