pexecute.c: Check HAVE_CONFIG_H, not IN_GCC, when determining whether to include...

pexecute.c: Check HAVE_CONFIG_H, not IN_GCC, when determining whether to include config.h.

        * pexecute.c: Check HAVE_CONFIG_H, not IN_GCC, when determining
        whether to include config.h.  Possibly include unistd.h in the
        !IN_GCC case.  Define VFORK_STRING as a printable function call
        for error messages (either "vfork" or "fork".)  If HAVE_VFORK_H is
        defined, include vfork.h.  If VMS is defined, define vfork()
        appropriately.  Remove vfork check on USG, we're using autoconf.
        (pexecute): Set `errmsg_fmt' to VFORK_STRING instead of checking
        locally what string to use.

From-SVN: r23499