Add variables defined in a list to add_compile_definitions() using cmake

Question

I have a list of vars definedinside a cmake which is called using from main CMakeLists.txt:

add_custom_target(gen_a
   COMMAND ${CMAKE_COMMAND} -P "gen_sigs.cmake"
)
add_dependencies(${PROJECT_NAME} gen_a)

My gen_sigs.cmake:

set(SIG_LIST SIGNAL_A=0 SIGNAL_B=1 SIGNAL_C=0 )

I want to use this list to add preprocessor definitions to the compiler command line like (in main CMakeLists.txt):

add_compile_definitions( 
  SIGNAL_A=0 
  SIGNAL_B=1
  SIGNAL_C=0
)

I tried using the following but it does not work (in main CMakeLists.txt):

add_compile_definitions( 
  ${SIG_LIST}
)

I am new to cmake so any suggestions on how to make this work will be helpful. Also the add_custom_target is defined before the add_compile_definitions() still it doesn't help. My guess is there is some problem with the order of execution because the ${SIG_LIST} has no value when I am printing it in cmakelists.txt. Is there a way to add dependency to add_compile_definitions so that my add_custom_target command gets forcibly executed before the add_compiler_definitions()?