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Now configure looks for it as an installed program instead.

Now we have variables for whether or not to build the docbook docs as
HTML, as PS, and as PDF. The configure script output now matches what
the build system will do (except it cannot take account of any
mk/build.mk settings, of course).

We use the GHC version number, as the old runghc one doesn't seem very
useful.

Patch from sthibaul. Fixes trac #2883.

We get caught by
    http://savannah.gnu.org/bugs/index.php?1516
    $(eval ...) inside conditionals causes errors
with make 3.80, so warn the user if it looks like they're about to
try to use it.

The original patch said:

* I made some changes to the way in which eta expansion happens in
  CorePrep, mainly to ensure that *arguments* that become let-bound
  are also eta-expanded.  I'm still not too happy with the clarity
  and robustness fo the result.
  
Unfortunately this change apparently broke some invariants that were
relied on elsewhere, and in particular lead to panics when compiling
with profiling on.

Will re-investigate in the new year.

We now use the in-tree haddock, so we don't need to look for it.

This allows us to put the proper version number into them

We shouldn't need it, as we don't call cygwin's gcc, and it was causing
problems with the nightly builders passing it to GHC.

 - remove $(ghc_ge_601), $(ghc_ge_602), $(ghc_ge_603)
 - configure now checks the GHC version number
 - there are probably various cleanups that we can now do in compat/
   and compiler/, but I haven't done those yet.