obstack: Fix portability issues

obstack: Fix portability issues

i686-apple-darwin10-gcc-4.2.1 (GCC) 4.2.1, SunOS 5.10, and possibly
others do not have exit.h and exitfail.h. Remove the use of these in
obstack.c.

The __block variable was renamed to block to avoid a gcc error:

compat/obstack.h:190: error: __block attribute can be specified on variables only

Initial-patch-by: David Aguilar <davvid@gmail.com>
Reported-by: Brian Gernhardt <brian@gernhardtsoftware.com>
Signed-off-by: Fredrik Kuivinen <frekui@gmail.com>
Acked-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>