FreeBSD and NetBSD share the same library execinfo.
This should really be replaced with a cmake-style check for where to find backtrace.
cmake 3.0 added a built-in package to handle this, but I think we still support an older cmake:
Since this is how the cmake build finds this, we don't need to worry about having other build systems here and we can do this in the correct way for cmake.
Can we do this later, after merging this change at it is? For now I won't be able to test the new approach.
We have got CMake 2.8 set as the minimal requirement.
I'm willing to push NetBSD bits first and then refactor for all platforms.
Are you OK with it @brucem?