CMake: No FFTW Needed for SYCL anymore

We do not need FFTW3 anymore to do FFTs on SYCL GPUs.

cmake/dependencies/FFT.cmake

@@ -48,14 +48,20 @@ if(ABLASTR_FFT)
  #
 
  # cuFFT (CUDA)
- # TODO: check if `find_package` search works
+ if(WarpX_COMPUTE STREQUAL HIP)
+ # nothing to do (cuFFT is part of the CUDA SDK)
+ # TODO: check if `find_package` search works for cuFFT
 
  # rocFFT (HIP)
- if(WarpX_COMPUTE STREQUAL HIP)
+ elseif(WarpX_COMPUTE STREQUAL HIP)
  find_package(rocfft REQUIRED)
 
- # FFTW (NOACC, OMP, SYCL)
- elseif(NOT WarpX_COMPUTE STREQUAL CUDA)
+ elseif(WarpX_COMPUTE STREQUAL SYCL)
+ # nothing to do (oneMKL is part of oneAPI)
+ # TODO: check if `find_package` search works for oneMKL
+
+ # FFTW (NOACC, OMP)
+ elseif()
  # On Windows, try searching for FFTW3(f)Config.cmake files first
  # Installed .pc files wrongly and unconditionally add -lm
  # https://github.com/FFTW/fftw3/issues/236