Update: User error! I missed the line in the description of the Windows environment that stated Git Bash is the shell used o run the builds. 
When building with our Makefile on Windows we use the standard command rmdir /Q /S to delete a directory. i.e.
clean-docs:
rmdir /Q /S docs
This works as expected when the make is run on a real Windows 10 laptop. However, when the make is run in a Travis Windows environment the delete step fails, seemingly because the /Q and /S options are being treated as directories…
rmdir /Q /S docs
The command "make build test verify-docs" exited with 2.
Done. Your build exited with 1.
rmdir: failed to remove '/Q': No such file or directory
rmdir: failed to remove '/S': No such file or directory
rmdir: failed to remove 'docs': Directory not empty
make: *** [Makefile:85: clean-docs] Error 1
Has anyone else seen this behaviour? Is there a known workaround?
Thanks,
George