Since R does not easily support subpackages/submodules, the
deterministic `evaluate` function is renamed `evald`, and the newly
created probabilistic one is named `evalp`. In order to make compile
the source files (*.cpp) in the probabilistic part of evalhyd, the
Makevars files needed to do a bit more work to find the source files
in the evalhyd dependency and place the object files (*.o) in the *src*
directory for the R package building to find them.