In particular, I'm surprised at the Darwin configurations apparently
not defining NO_IMPLICIT_EXTERN_C, and at most OpenBSD configurations
not doing so (but alpha-openbsd gets it from alpha/alpha.h); VxWorks
configurations are also inconsistent in this regard
(powerpc-wrs-vxworks gets it from rs6000/sysv4.h). If
NO_IMPLICIT_EXTERN_C were defined in elfos.h that would massively
reduce the number of targets which do not define it (and therefore the
number needing changing to replace NO_IMPLICIT_EXTERN_C by an
IMPLICIT_EXTERN_C macro with the opposite sense).