race condition in dune-2.8 python builder

I tried using dune python on an NFS share. After code generation we try to build the file, but sometimes cmake didn't yet realize that CMakeLists.txt has changed (as the changes are not yet fully synced).

What helped in my case was to add an os.sync() at the the beginning of the compile() function.

I didn't check yet, how the situation is on the development branch.