build: IS_TTY is now set according to GNU Make's MAKE_TERMOUT

build: IS_TTY is now set according to GNU Make's MAKE_TERMOUT

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Fixes: FS#2086
The logic for IS_TTY was broken, because it was testing stdin
instead of stdout.
MAKE_TERMOUT was introduced in GNU Make version 4.1 (05 Oct 2014),
so it should be available everywhere nowadays.

Signed-off-by: R. Diez <rdiezmail-openwrt@yahoo.com>