first cut at configurations patch, and a couple more

[Simon Marlow]

Simon Marlow wrote:
> Ok, spurned on by the possibiliy of worse merges to come, I've cleaned 
> up my current code, merged it up to date, and made it work (where "work" 
> means "doesn't make anything worse" - i.e. there is no new functionality 
> exposed to the user yet).

A little bit more about configurations.  Here's what you can do with them:

(a) enable some compiler options if --enable-debug is given to 'setup configure'

configuration: debug
ghc-options: -DDEBUG -O0

configuration: !debug
ghc-options: -O2

(b) depend on HUnit if --enable-debug is given:

configuration: debug
build-depends: HUnit-1.0

(c) if the base package is <2.0, depend on fps:

configuration: package(base-any, <2.0)
build-depends: fps>=1.0
ghc-options: -DUSE_FPS

(d) build executable E only if package P is available.  Actually I'm not really 
sure we want this, the executable could always go in a separate package. 
Anyway, if we want it, perhaps something like:

configuration: !package(P)
disable-executable: E

(e) Change dependencies depending on the operating system:

configuration: os(windows)
build-depends: Win32

configuration: !os(windows)
build-depends: unix


The conditional in a "configuration:" field has the following syntax:

   cond ::= STRING
          | cond || cond
          | cond && cond
          | !cond
          | 'package'(dependency)
          | 'package'(dependency, version-range)
          | 'os'(STRING)
          | 'arch'(STRING)

Cheers,
	Simon