Make gengetopt descriptions work.

Make gengetopt descriptions work.

With gengetopt's text and description options it is not possible to
print a text only when --detailed-help was given, but not when only
--help is given.

Moreover, headers generated with old versions of gengetopt do not
export the description text.  However, declaring it unconditionally
causes compiler warnings on new systems.

To circumvent these problems, we introduce a new test for configure
which checks whether the description string is declared. If it is not,
we simply append the declaration to the *.cmdline.h files.

This change allows to move the description string of para_play to
the gengetopt source file.