make CUDA_COMPUTE_BUILD_PATH use a relative path to the current binary directory

CUDA_COMPUTE_BUILD_PATH creates a directory for intermerdiate files based on the path of the source file being built relative to the current source directory.
This patch makes it use a path relative to the current binary directory in order to minimize path length.

Also see bug #14043
http://www.cmake.org/Bug/view.php?id=14043