The upstream Makefile uses DEBUG to control both stripping of debug
information *and* disabling optimisations.
We want to disable stripping of debug information so that (normally)
dh_strip can move it into a separate package later. However we also
want to enable optimisations by default.
Therefore:
- Set the STRIP variable (a command prefix) to "true"
- Set the DEBUG variable (a pseudo-boolean) to "true" if and only if
the "noopt" build option is used