install: fix SC20{04,28,86} warnings in configure

Author: xdelaruelle

configure

@@ -136,9 +136,9 @@ nagelfaraddons=y
 # check if output/error channels are attached to terminal
 for i in 1 2; do
    if [ -t $i ]; then
-      fdisterm[$i]=1
+      fdisterm[i]=1
    else
-      fdisterm[$i]=0
+      fdisterm[i]=0
    fi
 done
 
@@ -147,7 +147,7 @@ sgr() {
    local fd=$1
    local code=$2
    local msg=$3
-   if [ ${fdisterm[$fd]} -eq 1 ]; then
+   if [ "${fdisterm[fd]}" -eq 1 ]; then
       local out="\033[${code}m${msg}\033[0m"
    else
       local out=$msg
@@ -182,6 +182,7 @@ echo_warning() {
 
 # print usage message
 echo_usage() {
+   # shellcheck disable=SC2028
    echo "Usage: $progpath [OPTION]...
 Check requirements then set variables and create Makefiles.