Bugfix for previous pull request (Eigen)

Last pull request broke the webdemo. This PR addresses this issue.

@atgeirr Can you take a quick look? The CMake scripts in general are a bit poorly designed (some variables being set up in confusing manners), but this runs well with the webdemo on my machine now at least.
This don't compile on my system, but using 
```find_package(Boost REQUIRED COMPONENTS unit_test_framework )```
instead fixes the issue.