cmake FFTW_HAS_THREADS examples

ossia/score src/app/CMakeLists.txt :195 

check_symbol_exists(fftw_make_planner_thread_safe ${FFTW_LIBRARY} FFTW_HAS_THREADS)

ossia/score src/app/CMakeLists.txt :205 

target_compile_definitions(${APPNAME} PRIVATE FFTW_HAS_THREADS=1)