message: remove false error when enabling dpcpp manually
diff: -if (NOT GKO_CAN_COMPILE_DPCPP)
  - message(FATAL_ERROR "The CMAKE_CXX_COMPILER compiler, which is "
  - "${CMAKE_CXX_COMPILER} cannot compile DPC++ code!")
  -endif()
  -
  ginkgo_extract_dpcpp_version(${CMAKE_CXX_COMPILER} GINKGO_DPCPP_VERSION)
  set(GINKGO_DPCPP_VERSION ${GINKGO_DPCPP_VERSION} PARENT_SCOPE)
  