CMake: Avoid bogus preprocessor definitions
CMake: Avoid bogus preprocessor definitions

The macro CUDA_WARP_SRCS expects the target to have a set of
preprocessor definitions that does not contain an empty element.

danshapero/dealii,danshapero/dealii,sairajat/dealii,kalj/dealii,sairajat/dealii,kalj/dealii,EGP-CIG-REU/dealii,naliboff/dealii,sairajat/dealii,sairajat/dealii,danshapero/dealii,EGP-CIG-REU/dealii,naliboff/dealii,EGP-CIG-REU/dealii,kalj/dealii,naliboff/dealii,danshapero/dealii,EGP-CIG-REU/dealii,danshapero/dealii,kalj/dealii,naliboff/dealii,EGP-CIG-REU/dealii,naliboff/dealii,kalj/dealii,EGP-CIG-REU/dealii,kalj/dealii,kalj/dealii,sairajat/dealii,EGP-CIG-REU/dealii,naliboff/dealii,danshapero/dealii,danshapero/dealii,sairajat/dealii,sairajat/dealii,naliboff/dealii