Commit 003b33a8 recently added a call to basename().  On IRIX 6.5, this
function resides in libgen and -lgen is required for the linker.

Update configure.ac too.
Signed-off-by: NBrandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>