Give a useful message if autoconf is not installed

util/build.sh

@@ -11,6 +11,12 @@ if ! [ -x "$(command -v cmake)" ]; then
   exit 1
 fi
 
+if ! [ -x "$(command -v autoreconf)" ]; then
+  echo 'Error: autoconf is not installed. Install autoconf and try again.' >&2
+  echo 'On Debian-like systems: apt install autoconf' >&2
+  exit 1
+fi
+
 function cmd_pref() {
     if type -p "$2" > /dev/null; then
         eval "$1=$2"