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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>