Add env REMAKE_PS for custom color prompt

[yurenchen000]

#173

export REMAKE_PS=$'\033[1;35mremake\033[0m'

# readline compatible prompt (just enclosing the console sequences with \001 \002)
export REMAKE_PS=$'\001\033[1;33m\002remake\001\033[0m\002'

[rocky]

Thanks for the improvement!

[rocky]

Don't forget that there is a 4-4 branch tracking GNU make version 4.4.

I've applied this patch there as well.