Travis clang compilation time

[abdullah]

Taking a look at the time that clang takes to compile in Travis:
https://travis-ci.org/FreeCAD/FreeCAD/pull_requests

I see that:
- Some times, clang is even slower than the windows build:
https://travis-ci.org/FreeCAD/FreeCAD/builds/549000773

- However, other times it is extremely faster:
https://travis-ci.org/FreeCAD/FreeCAD/builds/548073084

In any case, it is in average substantially slower than gcc.

I was wondering if there is a reason (ccache not adapted), or if, maybe, there is something we can do to make it faster.