syntax error before 'cid_t'

[Ed L.]

Can anyone help me understand why this one-line program doesn't
compile under gcc 4.0.2, but does under HP ansi C compiler?
This is happening across several new HPUX 11.23 ia64 boxes...

$ uname -a
HP-UX oc5 B.11.23 U ia64 1725568875 unlimited-user license

$ cat s.c
#include <sys/ipc.h>

$ gcc -c s.c
In file included from s.c:1:
/usr/include/sys/ipc.h:51: error: syntax error before 'cid_t'
/usr/include/sys/ipc.h:56: error: syntax error before '}' token

$ cc -V
cc: HP aC++/ANSI C B3910B A.06.05 [Jul 25 2005]

$ cc -c s.c  
(no errors)

$ ls -l s.c
-rw-r--r--   1 root       sys             21 Jan 25 10:34 s.c

Ed