ENH: command-line query for the OPENFOAM_PLUS wmake value (issue #378)

Examples,

    wmakePrintBuild -plus

Check if value is known
(ie, everything configured and also OpenFOAM+):

    if wmakePrintBuild -plus >/dev/null 2>&1
    then
        echo YES
    else
        echo NO
    fi

Check if version is new enough

    if ofver=$(wmakePrintBuild -plus 2>/dev/null) && [ "$ofver" -ge 1612 ]
    then
        echo YES
    else
        echo NO
    fi

Conditionals

    ofver=$(wmakePrintBuild -plus 2>/dev/null)
    case "${ofver:=0}" in
    1612)
        echo "something for 1612
        ;;
    1706)
        echo "something for 1706
        ;;
    esac