unnecessary? #warning in sys/ipc.h

[Zack Weinberg]

sysvipc/sys/ipc.h contains this:

#if !defined __USE_MISC && !defined __USE_XOPEN && __GNUC__ >= 2
# warning "Files using this header must be compiled with _GNU_SOURCE
or _XOPEN_SOURCE"
#endif

It was added in 1998 (219aa9e9f9677e2f50095433ca233abdef23a6b0). with
no explanation, and as far as I can tell, is completely unnecessary;
the header (and the other sysvipc/ installed headers) work just fine
under e.g. -std=c11 with no feature selection macros (that is, they
declare all the things they're supposed to, and they don't trigger
compilation errors).

What do y'all think of removing it?

zw