In diagnosing a VSX-PCTS failure, one header-file-analysis test case was
not able to find <stddef.h>.  Further examination led to the fact that
there is NO <stddef.h> on the file system.  Admittedly, this does no
result in a compilation error, but does result in a standards-conformance
error.

$ pwd
/usr/include
$ grep -l stddef.h *.h
_G_config.h
alloca.h
curses.h
dirent.h
dlfcn.h
fmtmsg.h
gconv.h
glob.h
grp.h
iconv.h
libiberty.h
libintl.h
locale.h
malloc.h
ncurses.h
netdb.h
obstack.h
printf.h
pwd.h
regex.h
search.h
shadow.h
stdint.h
stdio.h
stdlib.h
string.h
strings.h
time.h
tk.h
unistd.h
wchar.h
wctype.h
zutil.h