message: add CMake cache variable for injecting arbitrary extra flags for GEN
diff: @@ -223,6 +223,8 @@ find_program(ISPC_EXECUTABLE_GPU ispc-gpu DOC "Path to GEN enabled ISPC.")
  define_ispc_isa_options(GEN genx-x8 genx-x16)
  +set(ISPC_GENX_ADDITIONAL_ARGS "" CACHE STRING "extra arguments to pass to ISPC for GEN targets")
  +
  macro (ispc_compile_gpu parent_target output_prefix)
  if(ISPC_INCLUDE_DIR)
  string(REPLACE ";" ";-I;" ISPC_INCLUDE_DIR_PARMS "${ISPC_INCLUDE_DIR}")
  @@ -291,12 +293,12 @@ macro (ispc_compile_gpu parent_target output_prefix)
  -I ${CMAKE_CURRENT_SOURCE_DIR}
  ${ISPC_INCLUDE_DIR_PARMS}
  ${ISPC_GENX_OPT_FLAGS}
  - ${ISPC_GENX_ADDITIONAL_ARGS}
  -DISPC_GPU
  --addressing=64
  --target=${ISPC_TARGET_GEN}
  ${ISPC_GPU_OUTPUT_OPT}
  --woff
  + ${ISPC_GENX_ADDITIONAL_ARGS}
  -o ${result}
  ${input}
  COMMENT "Building ISPC GPU object ${result}"
  