You can use specialized properties for some configurations: IMPORTED_IMPLIB_<CONFIG> and IMPORTED_LOCATION_<CONFIG>.
SET_PARGET_PROPERTIES(
ExternFMOD
PROPERTIES
IMPORTED_IMPLIB_DEBUG "${_fmod_lib_dir}/fmodL64_vc.lib"
IMPORTED_IMPLIB "${_fmod_lib_dir}/fmod64_vc.lib"
IMPORTED_LOCATION_DEBUG "${_fmod_lib_dir}/fmodL64.dll"
IMPORTED_LOCATION "${_fmod_lib_dir}/fmod64.dll"
)