If _XOPEN_SOURCE_EXTENDED or _XOPEN_SOURCE are defined, _POSIX_C_SOURCE gets

If _XOPEN_SOURCE_EXTENDED or _XOPEN_SOURCE are defined, _POSIX_C_SOURCE gets
defined in DECC$TYPES.H.  If _POSIX_C_SOURCE is defined, certain types do
not get defined (u_char, u_int, ...).  DECC.H gets included by assert.h
and others.  Now, in6.h uses the types u_char, u_int and so on, and gets
included as part of other header inclusions, and will of course fail because
of the missing types.

On the other hand, _XOPEN_SOURCE_EXTENDED is needed to get gethostname()
properly declared...

Solution: define _XOPEN_SOURCE_EXTENDED much later, so DECC$TYPES.H has
a chance to be included *first*, so the otherwise missing types get defined
properly.

Personal: *mumble* *mumble*