Summary:
Remove some Windows specific code from the .m4 files
and have configure figure it out.

Unfortunately touchy can't be removed since there
is no mingw build of coreutils. Only msys builds
which would give us a dependency on the msys runtime.

Reviewers: hvr, austin, thomie, bgamari

Reviewed By: thomie, bgamari

Subscribers: thomie, erikd, #ghc_windows_task_force

Differential Revision: https://phabricator.haskell.org/D2248