gcc 2.95.1 problem with 64-bit off_t and __STDC_EXT__ in HP-UX 10.20

[Paul Eggert <eggert at twinsun dot com>]

Paul D. Smith reports that GNU make 3.77.95 doesn't build with GCC 2.95.1
on HP-UX 10.20, with the following failure symptoms:

  gcc10 -DHAVE_CONFIG_H -I. -I../../glob -I..   -I../glob  -O2 -c ../../glob/glob.c
  In file included from /usr/include/errno.h:6,
                   from ../../glob/glob.c:32:
  /usr/include/sys/stdsyms.h:244: #error "Large Files (ILP32) not supported in strict ANSI mode."

We worked around the problem by compiling with -D__STDC_EXT__.

Dejanews reports that people have run into similar problems building
Samba and Berkeley DB on HP-UX 10.20 and later with gcc; they've
worked around the problem similarly.

I believe that the problem is that /usr/include/sys/stdsyms.h contains
the following code:

  #if defined(__STDC_EXT__) || !defined(__STDC__) || defined(__LP64__)
    ...
  #else  /* strict ANSI */

  # ifdef  _FILE_OFFSET_BITS
  #  if !defined(__LP64__) && _FILE_OFFSET_BITS == 64
      #error "Large Files (ILP32) not supported in strict ANSI mode."
  #  endif
  # endif

  # ifdef _LARGEFILE64_SOURCE
      #error "Large File interfaces not supported in strict ANSI mode."
  # endif

  #endif /* __STDC_EXT__ || ! __STDC__  || __LP64__ */

GNU `make' needs access to large files, so it sets _FILE_OFFSET_BITS
to 64; this runs afoul of the above check with gcc 2.95.1, but not
with the HP compiler.

I suggest that GCC be modified on one of two ways:

1) Define __STDC_EXT__ as a builtin macro in HP-UX 10.20 and later,
   unless --ansi is given.

or

2) Do not define __STDC__ in system headers in HP-UX 10.20 and later,
   unless --ansi is given.  This would be similar to the practice in
   Solaris, where __STDC__ is 0 in system headers; it would differ in
   that __STDC__ would be _undefined_ in system headers.

(2) sounds more plausible to me, but unfortunately I don't have access
to an HP-UX 10.20 or later box to test it.  However, I thought I'd
report the situation as best I could; perhaps someone with access to
HP-UX 10.20 or later can take it from here.