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: NClément Chigot <clement.chigot@atos.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

config.mak.uname

@@ -269,6 +269,7 @@ ifeq ($(uname_S),AIX)
 	INTERNAL_QSORT = UnfortunatelyYes
 	NEEDS_LIBICONV = YesPlease
 	BASIC_CFLAGS += -D_LARGE_FILES
+	FILENO_IS_A_MACRO = UnfortunatelyYes
 	ifeq ($(shell expr "$(uname_V)" : '[1234]'),1)
 		NO_PTHREADS = YesPlease
 	else