If you are installing the package from a .tar.gz file, that archive contains all the necessary files. However, if you are working directly from the git repository, some of the necessary files are in other repositories and we need to create symbolic links to those files within this. This only needs to do once.
To work with the git repository, you will need to create symbolic links to RConverters.c and RConverters.h from the RAutoGenRunTime package or repository. You might do something like
git clone https://github.com/duncantl/RCUDA
git clone https://github.com/omegahat/RAutoGenRunTime
cd RCUDA/src
ln -s ../../RAutoGenRunTime/src/RConverters.c .
ln -s ../../RAutoGenRunTime/inst/include/RConverters.h .
ln -s ../../RAutoGenRunTime/inst/include/RError.h .