Proposal: Add --with-libedit flag to the readline package

[Judah Jacobson]

It would be useful for the readline package to support compiling
against the libedit library (which provides a subset of the readline
APIs):

 - libedit is available by default on OS X.
 - since libedit is BSD-licensed, there are no problems statically
linking ghc with it. (This may be useful on Windows.)

I propose adding a --with-libedit flag to the readline autoconf
script. Without that flag, the package will behave exactly as before,
refusing to link against libedit. With that flag, the following
behavior occurs:

 - GNUreadline.framework (OS X - only) is ignored, if present
 - We try to link with -lreadline, and don't fail if readline is
actually libedit.
 - If it is libedit, we #ifdef out all of the functions not supported
by libedit. (these are generally low-level APIs not needed by most
applications, including ghci.) Otherwise, if we're linking against GNU
readline, we support all the available APIs.

Suggested deadline: Jan. 23, 2008.  A patch is attached to the trac
ticket: http://hackage.haskell.org/trac/ghc/ticket/2028

Thoughts?

-Judah