[Cmake] Adding a debug switch with CMake

[Neil Killeen]

In fact I think there is a whole pile of things I would
like to do in this way without having to reconfigure CMake
and regenerate makefiles

E.g.

make debug
make verbose

etc and then you might even like to add your own arguments
on the command line

make CMAKE_CXX_FLAGS+="-O1"

and so on.

Perhaps the cross-platform requirements on CMake make
this kind of approach intractable ?

cheers
Neil