bugfix when ispc not installed

Author: svenwoop

common/cmake/ispc.cmake

@@ -20,7 +20,9 @@ MACRO (INCLUDE_DIRECTORIES_ISPC)
 ENDMACRO ()
 
 execute_process(COMMAND which ispc OUTPUT_VARIABLE ISPC_EXECUTABLE)
-execute_process(COMMAND dirname ${ISPC_EXECUTABLE} OUTPUT_VARIABLE ISPC_DIR_TEMP)
+IF (EXISTS ${ISPC_EXECUTABLE})
+  execute_process(COMMAND dirname ${ISPC_EXECUTABLE} OUTPUT_VARIABLE ISPC_DIR_TEMP)
+ENDIF()
 STRING(REGEX REPLACE "\n" "" ISPC_DIR "${ISPC_DIR_TEMP}")
 
 MACRO (ispc_compile targets)