Setup ccache for better workflow with multiple branches

[Fuco1]

In the wake of the humongous branches I think it would be a good idea to keep them smaller and more contained.

One problem that I often face is that it takes a lot of time recompiling everything when switching branches. There's a solution called ccache among others that can cache the compilation results and speed up the process significantly.

Here's an article on how to set it up with regular configure/makefile setup: http://jsteemann.github.io/blog/2015/02/07/using-ccache-when-working-with-different-branches/

I don't know where the compile commands are set in cmake but all that has to be done is basically invoke ccache g++ instead of g++.

[wheybags]

Looks like this is a pretty simple one-liner: https://stackoverflow.com/a/37828605

[wheybags]

It would also be nice to have https://github.com/frerich/clcache for windows too, and especially if we could use both ccache and clcache on travis/appveyor