Extending the dependency syntax

[Simon Marlow]

I'd like to wrap up this little design issue which stalled.  I'll
propose something a little simpler, which should hopefully be closer to
what people are used to from other package systems, and which should
translate more easily.

To recap, the goal is to support:

  - build options that result in an executable or library being built
    in a different way, or with a particular feature.  The build option
    may imply additional dependencies.

  - a choice between dependencies, where the particular choice made
    dictates certain configuration changes in the build info.

  - a choice between configurations, for example building a library
    with or without debugging support.

An example first showing all the main ideas:

------------------
build-depends:
	  ghc? (ghc >= 6.4, [ghc64] | ghc >= 5.04, [ghc-old]) 
	   | hugs? [hugs],
        debug? 
           (HUnit-1.0, [debug]) 
           [release],
        gnome? ( libglade >= 2,
                 gtksourceview >= 0.6,
                 gconf >= 2, [gnome] ),
        mozilla? ( mozilla >= 1.4, [mozilla] ),
        doc? ( haddock >= 0.6 )

[gnome]
extra-libraries: gnome
extra-ghc-options: -DENABLE_GNOME
...

[debug]
extra-ghc-options: -O0 -DDEBUG

[release]
extra-ghc-options: -O2

-------------------

The semantics of build-depends:

  - ',' means "and", just as it does now
  - '|' means "or"
  - parentheses group dependencies
  - [name] means "read the optional stanza [name]"
  - 'flag? dep' means "if the --enable-flag option is given, then
    dep, otherwise nothing". 
  - 'flag? dep1 dep2' menas "if the --enable-flag option is given
    then dep1, else dep2".

Additionally, we could have an --auto-enable flag, which has the effect
of replacing every 'flag? dep' by 'dep|', and 'flag? dep1 dep2' by
'dep1|dep2'. This has the effect of turning on every option for which
the dependencies are satisfied, which is normally what you want when
building a package for local use.

I'm not attached to any particular syntax, so feel free to suggest
alternatives.

This is somewhat simpler than before, and I think supports everything we
need.  Comments?  I'm happy to implement if there's a concensus.

Cheers,
	Simon