(__GNUC_PREREQ, always_inline, inline, pure):

(__GNUC_PREREQ, always_inline, inline, pure):
Remove.
(__attribute): Define to empty unless GCC 3.1 or later.
This works around a core dump on OpenBSD 3.4, which has GCC
2.95.3, which dumps core when given __attribute__(()).  It also
simplifies other tests, since we really don't want to bother with
worrying about which ancient version of GCC supported what.
Original problem reported by Yoann Vandoorselaere, with part of
the fix suggested by Derek Price.