Actually, a better way for -portable software to handle this might be
to do this at a higher level:

#ifndef F_BELOW
#define F_BELOW F_GETFL
#endif

This will return a value which isn't -1 with errno, and can be
safely ignored.  So either (void) or non-(void) constructs will work.
In that case I don't need any of these #ifndef in the tree.