Build stalling during clang 6 compile

This build just cowardly stalls in the compilation part:

https://travis-ci.org/adrpar/tin-terrain/jobs/445664813

I have tried many different things to resolve this, but have not found anything that works. Any ideas? It is not even the last file that should be compiled, there is another one after the one where it stalls.

Thanks for your help.

Adrian