append FFT libraries only if found

CMakeLists.txt

@@ -327,12 +327,18 @@ if (MORSE_DISTRIB_DIR OR EXISTS "${CMAKE_SOURCE_DIR}/CMakeModules/morse/")
         # MKL, THREADS|OMP and/or SIMPLE|DOUBLE|LONG|QUAD
         # Default is DOUBLE and without THREADS|OMP
         find_package(FFTW  COMPONENTS MKL) # not REQUIRED
-        set(FFT_LIBRARIES "-L${FFTW_LIBRARY_DIRS_DEP};" CACHE STRING "Set your MKL flags")
-        foreach (fft_lib ${FFTW_LIBRARIES_DEP})
-          set(FFT_LIBRARIES "${FFT_LIBRARIES} ${fft_lib};")
-        endforeach()
+        if (FFTW_LIBRARY_DIRS_DEP)
+            set(FFT_LIBRARIES "-L${FFTW_LIBRARY_DIRS_DEP};" CACHE STRING "Set your MKL flags")
+        endif()
+        if (FFTW_LIBRARIES_DEP)
+            foreach (fft_lib ${FFTW_LIBRARIES_DEP})
+              set(FFT_LIBRARIES "${FFT_LIBRARIES} ${fft_lib};")
+            endforeach()
+        endif()
         set(FFT_INCLUDES "${FFTW_INCLUDE_DIRS_DEP}" )
-        set(SCALFMM_LIBRARIES "${SCALFMM_LIBRARIES}; ${FFT_LIBRARIES}")
+        if (FFT_LIBRARIES)
+            set(SCALFMM_LIBRARIES "${SCALFMM_LIBRARIES}; ${FFT_LIBRARIES}")
+        endif()
 
       endif(SCALFMM_USE_MKL_AS_BLAS)