PP/35 Pull in <features.h> on Linux, for some portability edge-cases of
64-bit ${eval} (JH/03).
-PP/36 Define _ISOC99_SOURCE in exim.h; it's needed for some releases of
+PP/36 Define _GNU_SOURCE in exim.h; it's needed for some releases of
GNU libc to support some of the 64-bit stuff, should not lead to
conflicts. Defined before os.h is pulled in, so if a given platform
needs to override this, it can.