build/prereq: require make 4.1 or later

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>

README.md

@@ -25,7 +25,7 @@ the [Build System Setup](https://openwrt.org/docs/guide-developer/build-system/i
 documentation.
 
 ```
-gcc binutils bzip2 flex python3 perl make find grep diff unzip gawk getopt
+gcc binutils bzip2 flex python3 perl make4.1+ find grep diff unzip gawk getopt
 subversion libz-dev libc-dev rsync
 ```
 

include/prereq-build.mk

@@ -11,8 +11,8 @@ PKG_NAME:=Build dependency
 
 # Required for the toolchain
 $(eval $(call TestHostCommand,working-make, \
-	Please install GNU make v3.82 or later. (This version has bugs), \
-	$(MAKE) -v | grep -E 'Make (3\.8[2-9]|3\.9[0-9]|[4-9]\.)'))
+	Please install GNU make v4.1 or later., \
+	$(MAKE) -v | grep -E 'Make (4\.[1-9]|[5-9]\.)'))
 
 $(eval $(call TestHostCommand,case-sensitive-fs, \
 	OpenWrt can only be built on a case-sensitive filesystem, \