cmake: drop FFTW2 support

if(ENABLE_KSPACE)

  set(FFT "KISSFFT" CACHE STRING "FFT library for KSPACE package")

  set_property(CACHE FFT PROPERTY STRINGS KISSFFT FFTW3 MKL FFTW2)

  set_property(CACHE FFT PROPERTY STRINGS KISSFFT FFTW3 MKL)

  if(NOT FFT STREQUAL "KISSFFT")

    find_package(${FFT} REQUIRED)

    add_definitions(-DFFT_${FFT})

# - Find fftw2

# Find the native FFTW2 headers and libraries.

#

#  FFTW2_INCLUDE_DIRS - where to find fftw2.h, etc.

#  FFTW2_LIBRARIES    - List of libraries when using fftw2.

#  FFTW2_FOUND        - True if fftw2 found.

#

find_path(FFTW2_INCLUDE_DIR fftw.h)

find_library(FFTW2_LIBRARY NAMES fftw)

set(FFTW2_LIBRARIES ${FFTW2_LIBRARY})

set(FFTW2_INCLUDE_DIRS ${FFTW2_INCLUDE_DIR})

include(FindPackageHandleStandardArgs)

# handle the QUIETLY and REQUIRED arguments and set FFTW2_FOUND to TRUE

# if all listed variables are TRUE

find_package_handle_standard_args(FFTW2 DEFAULT_MSG FFTW2_LIBRARY FFTW2_INCLUDE_DIR)

mark_as_advanced(FFTW2_INCLUDE_DIR FFTW2_LIBRARY )