The BoolOpt element defines a boolean option, i.e. an option that takes no
argument. It does not enclose any other elements. For example:
<BoolOpt letter="c" name="sample_bool"/>
For C, the option name will be used as the name of the corresponding member,
of type int, to be used as a boolean. For C++ the generated code will provide
two members:
A private boolean data member whose name is the name of the option, followed
by an underscore.
A const public member function returning the value of the data member. Its
name is the same as the name of the option.
