Build system changes

[Claus Reinke]

> Now merged into the HEAD.

great, thanks! on my win/xp setup (cygwin tools, mingw gcc),

    sh boot
    ./configure --host=i386-unknown-mingw32
    make
    make binary-dist

seems to produce a useable ghci.exe again, with ghc-pkg.exe
showing a more healthy listing as well. 

some issues:-)

- execute permissions are missing for ghcii.sh

- configure still doesn't normalize the gcc path (as it does for ld);
    since i only have one version of gcc/ld around, i omitted configure
    parameters --with-gcc and --with-ld, which almost works, but for
    this bit in mk/config.mk, which i reported earlier:

    WhatGccIsCalled = gcc
    ..
    GccDir = $(dir $(WhatGccIsCalled))

    unless either configure or mk/config.mk normalise the gcc path,
    the setting for GccDir will be wrong, which will cause trouble in
    the Makefile/install:: target.

- this exposes another issue, as there seems to be some overlap 
    in who copies what into the binary distribution. some of that
    is done in Makefile/install::, some in Makefile/binary-dist::, via
    distrib/prep-bin-dist-mingw. i suspect that all of this should be
    localised in prep-bin-dist-mingw?

claus