Makefile: use fileno macro work around on AIX
Declare FILENO_IS_A_MACRO on AIX
On AIX, fileno(fp) is a macro and need to use the work around already made for BSD's.
Signed-off-by: Clément Chigot <clement.chigot@atos.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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