[openwrt/openwrt] build: IS_TTY is now set according to GNU Make's MAKE_TERMOUT

[LEDE Commits]

aparcar pushed a commit to openwrt/openwrt.git, branch master:
https://git.openwrt.org/a015d9170851509a58e063dd82a054e98fb48700

commit a015d9170851509a58e063dd82a054e98fb48700
Author: R. Diez <rdiezmail-openwrt at yahoo.com>
AuthorDate: Wed Jun 10 11:33:26 2020 +0200

    build: IS_TTY is now set according to GNU Make's MAKE_TERMOUT
    
    The sender domain has a DMARC Reject/Quarantine policy which disallows
    sending mailing list messages using the original "From" header.
    
    To mitigate this problem, the original message has been wrapped
    automatically by the mailing list software.
    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 at yahoo.com>
---
 include/toplevel.mk | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/toplevel.mk b/include/toplevel.mk
index 744695b19c..48ea44aa24 100644
--- a/include/toplevel.mk
+++ b/include/toplevel.mk
@@ -4,7 +4,7 @@
 
 PREP_MK= OPENWRT_BUILD= QUIET=0
 
-export IS_TTY=$(shell tty -s && echo 1 || echo 0)
+export IS_TTY=$(if $(MAKE_TERMOUT),1,0)
 
 include $(TOPDIR)/include/verbose.mk