Added option to 'configure' to control debug information for compilation
As reported by Amadeusz Sławiński, there were a number of debug-only
information printed by the menu parser, which had no reason to be
in a user package.
This patch removes all the hard-coded DEBUG definitions pointed
by Amadeusz and adds a new (standard-like) option '--enable-debug'
to the configure script, which activates debug stuff for the devs.
The default behaviour is now to not have them.
As a side effect, the option also disable optimisation, which are
generally annoying when trying to run a debugger.