The header in question is using the standard (glibc) <sys/cdefs.h>.
It would be sufficient to include <bsd/bsd.h> anytime before header in question. Or redefine __CONCAT directly.
Do you think we can fix it there? (I.e. with upstream consentment)
Well, behaviour of glibc upstream is unpredictable ;-)
Btw, does __STRING need this fix too?
I don't think so. Petr