sextest resolves #+ and #- itself -- it reads the program and prints
what survives to sexc -- so a flag spelling it does not recognise
decides which branch gets compiled
'#+' 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.
