For bool valued flags we were always producing the command line
string corresponding to a false flag value, even if the flag was
not set. For example we'd always get "--disable-shared".
It is important for cabal-install to be able to take an empty set
of flags, override a few flags and turn the flags back into
command line strings without getting a lot of extra defaults.
Partly this is because we have to work with older versions of the
Cabal library command line which does not recognise the new options.