[Rivet] Force rivet to use its own yaml-cpp

[David Grellscheid]

Hi David,

> Thanks for your help. I suppose rivet 2.0 will suffer from future
> changes to the yaml-cpp API then?

It works with both :-)

> You're suggestion makes the build possible, but is not quite working.
> When I 'make install' it tries to overwrite the existing header files
> in /usr/local/include/yaml-cpp

Yes.

> Now I could just leave those uninstalled, but then other programs
> that build with rivet (e.g. Herwig++) might struggle to build, right?

No, no other codes will be affected, as YAML is not exposed in the Rivet 
header files. Only Rivet itself needs to be able to link against it. You 
could change the linking of Rivet to YAML to be static in the Makefile. 
Let us know if you need help with that.

   David