FS#2086 "IS_TTY in the makefile is broken" reports flawed detection of
stdout piping to a file. The issue describes how e.g. terminal color
codes and up in log files if running make like `make > log.txt`.

The proposed solution uses the make variable "MAKE_TERMOUT", which was
introduced in make 4.1. All major distributions seem to updated to 4.1
or later, so this ideally dosen't break anything.

Signed-off-by: Paul Spooren <mail@aparcar.org>