[!744] Fixes for mingw

Merge branch 'fixes-for-mingw' into 'master'

ref:core/dune-common We fix two general issues f the build-system regarding
mingw/windows support.

@gruenich and again\... can we get this into 2.7?

See merge request [!744]

  [!744]: gitlab.dune-project.org/core/dune-common/merge_requests/744