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: NBrandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: NGary V. Vaughan <gary@thewrittenword.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>