Install ffmpeg with --enable-opencl and boxblur_opencl filter

I need help to fix my FFmpeg and make it provide boxblur_filter.

I installed FFmpeg with beblow config.

ffmpeg version n4.2.4-7-gc9f3835b2b Copyright (c) 2000-2020 the FFmpeg developers
built with gcc 7 (Ubuntu 7.5.0-3ubuntu1~18.04)
configuration:
--prefix=/usr/local/ffmpeg --enable-shared --enable-cuda --enable-cuvid
--enable-nvenc --enable-nonfree --enable-libnpp --enable-filter=drawtext
--enable-libfreetype --enable-opencl --extra-ldflags=-L/usr/local/cuda/lib64
--extra-cflags=-I/usr/local/cuda/include


and I expect that FFmpeg provide boxblur_opencl filter.

https://ffmpeg.org/ffmpeg-filters.html#boxblur_005fopencl

but I only found avgblur_filter by FFmpeg -filters | grep OpenCL.

... avgblur_opencl V->V Apply average blur filter
... colorkey_opencl V->V Turns a certain color into transparency. Operates on RGB colors.
... convolution_opencl V->V Apply convolution mask to input video
... dilation_opencl V->V Apply dilation effect
... erosion_opencl V->V Apply erosion effect
... nlmeans_opencl V->V Non-local means denoiser through OpenCL
... overlay_opencl VV->V Overlay one video on top of another
... prewitt_opencl V->V Apply prewitt operator
... program_opencl |->V Filter video using an OpenCL program
... roberts_opencl V->V Apply roberts operator
... sobel_opencl V->V Apply sobel operator
... tonemap_opencl V->V perform HDR to SDR conversion with tonemapping
... transpose_opencl V->V Transpose input video
... unsharp_opencl V->V Apply unsharp mask to input video
... openclsrc |->V Generate video using an OpenCL program