Revise processing of -j flag

- Accept only integer numbers for -j flag
- Allow for -j as standalone flag

tools/toolchain/install_cp2k_toolchain.sh

@@ -374,7 +374,19 @@ while [ $# -ge 1 ]; do
   case ${1} in
     -j)
       shift
-      export NPROCS_OVERWRITE="${1}"
+      case "${1}" in
+        -*)
+          export NPROCS_OVERWRITE="$(get_nprocs)"
+          ;;
+        [0-9]*)
+          export NPROCS_OVERWRITE="${1}"
+          ;;
+        *)
+          report_error ${LINENO} \
+            "The -j flag can only be followed by an integer number, found ${1}."
+          exit 1
+          ;;
+      esac
       ;;
     -j[0-9]*)
       export NPROCS_OVERWRITE="${1#-j}"