Distribution.PreProcess

[Ross Paterson]

A couple of remarks about the current design:

Currently the output of preprocessing is left in the same directory as
the source.  For Hugs, it would be more convenient to preprocess a source
tree to make a new tree, especially if cpp turns Foo.hs into Foo.hs.

The restriction to extensions delimited with '.' rules out things like
*_hsc_make.c (portable output of hsc2hs).

It might be preferable not to unlit .lhs files, as all implementations
can handle them, and the originals are better for human readers (such
as Hugs users curious about their libraries).  There's no real need to
unlit .ly files either, though some other preprocessors will need it.
(And should unlit be treated as just another preprocessor?)

As I understand it, preprocessing with cpp is only needed for Hugs,
as the compilers have -cpp flags, and will do it properly themselves.
For Hugs, cpp will need a -D__HUGS__ option, plus extra options that
are often system-dependent (and will also be needed by the compilers).
This harks back to the need to separate system-dependent build parameters
from the package description, and provide hooks to generate and use them.