Updates to the build system

Following the discussion of the 16/09 it would be good to update our build system such that all the different choices of modules are made at configuration time without having to edit a file.

This should be handled by the autotools and default to a sensible set of parameters if the user does not specify anything. The content of const.h should then be limited to a set of constants.

This is very low priority but I'll add it here to keep track of things.