add GPU cleanup functionality

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
Makefile: remove C++ references

We don't build .cpp files anymore...

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
Makefiles: add dependency generation

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
Makefiles: when NO_CUDA is selected, just use the same flags as CC

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
nbody.cu: cull some dead code

Signed-off-by: Steven Noonan <steven@uplinklabs.net>