add librt for linux

f_fftw_spectrum/CMakeLists.txt

@@ -8,5 +8,8 @@ find_package(blc REQUIRED)
 
 add_executable(f_fftw_spectrum f_fftw_spectrum.cpp)
 target_link_options(f_fftw_spectrum PRIVATE -Wno-multichar)
-target_link_libraries(f_fftw_spectrum PRIVATE blc rt ${FFTW_LIBRARIES})
+target_link_libraries(f_fftw_spectrum PRIVATE blc ${FFTW_LIBRARIES})
 
+if(UNIX AND NOT APPLE)
+    target_link_libraries(f_fftw_spectrum PRIVATE rt)
+endif()