IRIX 6.5.26m does not define the 'sgi' macro, but it does define an '__sgi'
macro. Since later IRIX versions (6.5.29m) define both macros, and since
an underscore prefixed macro is preferred anyway, use '__sgi' to detect
compilation on SGI IRIX.
Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil> Signed-off-by: Gary V. Vaughan <gary@thewrittenword.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>