configure: make call to readlink portable to non-GNU (BSD) systems

The argument '-e' for readlink is a GNU extension.
In setting the variable 'where' the argument '-e' is not necessary, thus eliminated.

configure

@@ -160,7 +160,7 @@ check_for() {
                 printf "Checking for '${item}'... "
                 where="$( gcc -print-file-name="${item}" )"
                 if [ "${where}" != "${item}" ]; then
-                    where="$( readlink -e "${where}" )"
+                    where="$( readlink "${where}" )"
                     status=yes
                     break;
                 fi