'#+' and '#-' introduce conditional compilation: the form that follows
is kept only when the feature expression is true, and otherwise is read
and thrown away. An expression is a feature name, or and / or / not
of them.
They are read time, not compile time.
Default features are the host's software-version, software-type and
machine-type as CHICKEN reports them, plus what --features flag adds.
Also rename macros to sex-macros, module-system to sex-modules for
clarity, uniformity, and to avoid name clashes with Chicken's
units/modules named "macros" and "modules"
