configure: Throw error if OS is found to be msys
configure: Throw error if OS is found to be msys

Test Plan: Try running configure on Windows with msys and mingw
toolchains

Reviewers: Phyx, hvr

Reviewed By: Phyx

Subscribers: rwbarton, thomie, erikd, carter

Differential Revision: https://phabricator.haskell.org/D4526

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc