mi: Provide ffs when compiling using MinGW

I don't know how I feel about the promise made in 2e7f790b :)
4 jobs for mingw-fixes in 5 minutes and 25 seconds (queued for 1 second)
Status Name Job ID Coverage
  Docker Image
passed debian-testing #306823

00:00:28

 
  Build And Test
passed autotools-build-and-test #306824

00:04:57

passed meson-build-and-test #306825

00:01:44

 
  External
passed continuous-integration/appveyor/branch