sys/cdefs.h: Add comments to make #if/#else/#endif triple more obvious

This block has a lot of nesting, not helped by two adjacent nested
blocks involving _POSIX_C_SOURCE, with only the inner one commented,
looking like it's the end of the outer one. Comment the outer one as
well so it's not quite so hard to figure out.

MFC after: 1 week

(cherry picked from commit 7a3af393d8ac2dfe72d24fe401344b60c4f87866)