CacheInterface.h: make CkCache.h an angled #include

Needed for similar reason as described in commit @c9a512e: ChaNGa's
dependency generation assumes that CkCache.h is a local include, and here
the files _it_ includes were being listed without path in Makefile.dep.
Merge branch 'build-system-fixes' into cuda-project
Merge pull request #4 from insaneinside/build-system-fixes

CacheInterface.h: make CkCache.h an angled #include
cuda.ac, configure: only generate cuda.mk if CUDA is enabled
Makefile.in: keep C/C++ PreProcessor FLAGS (-D, -I, etc.) in CPPFLAGS
Makefile.in: corrected references to Makefile.dep (lives in SOURCE dir)
Makefile.in: separate include flags into a separate variable

Easier to avoid passing the configure-chosen `-fpic' flag to `nvcc`
this way (nvcc doesn't like it).  This should fix the problem I'm
having with compiling the CUDA branch on the Intel compiler suite.
cuda.ac: use $withval, not $enableval, in processing --with-cuda-level
Merge branch 'build-system-fixes' into cuda-project
Merge branch 'cuda-project' into cuda-scaledmoments