-with the definition in sysexits.h. Arrange to preserve it, even though at
-present Exim doesn't actually use it. */
+with the definition in sysexits.h. Exim does not actually use this macro, so we
+just undefine it. It would be nice to be able to re-instate the definition from
+sysexits.h if there is no definition in unistd.h, but I do not think there is a
+way to do this in C because macro definitions are not scanned for other macros
+at definition time. [The code here used to assume they were, until I was
+disabused of the notion. Luckily, since EX_OK is not used, it didn't matter.] */