Compiling on Ubuntu 16: undefined reference to VideoFrameTransform_new

Start by reproducing the Ubuntu 16.04 build with the reported ffmpeg v3.3.1 configuration and inspect libavfilter/vf_transform360.c together with the Transform360 library build and link instructions. Verify how the VideoFrameTransform_* symbols are provided and linked. Done means the make step completes and ffmpeg_g links without undefined references.

One of our researchers has asked us to compile this but I've run into an issue.

$ make
[...]
LD	ffmpeg_g
libavfilter/libavfilter.a(vf_transform360.o): In function `generate_map':
/local-scratch/swbuilder/Linux/Ubuntu/16.04/amd64/ffmpeg-3.3.1/libavfilter/vf_transform360.c:136: undefined reference to `VideoFrameTransform_new'
/local-scratch/swbuilder/Linux/Ubuntu/16.04/amd64/ffmpeg-3.3.1/libavfilter/vf_transform360.c:152: undefined reference to `VideoFrameTransform_generateMapForPlane'
libavfilter/libavfilter.a(vf_transform360.o): In function `filter_frame':
/local-scratch/swbuilder/Linux/Ubuntu/16.04/amd64/ffmpeg-3.3.1/libavfilter/vf_transform360.c:344: undefined reference to `VideoFrameTransform_transformFramePlane'
libavfilter/libavfilter.a(vf_transform360.o): In function `uninit':
/local-scratch/swbuilder/Linux/Ubuntu/16.04/amd64/ffmpeg-3.3.1/libavfilter/vf_transform360.c:295: undefined reference to `VideoFrameTransform_delete'
collect2: error: ld returned 1 exit status
Makefile:136: recipe for target 'ffmpeg_g' failed
make: *** [ffmpeg_g] Error 1

Distro: Ubuntu 16.04.2 LTS (Xenial Xerus)
ffmpeg source: v3.3.1
gcc (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609

transform360 itself compiles fine, but following the instructions at
https://github.com/facebook/transform360 yields the error above.

Troubleshooting steps I've tried:

• vf_transform360.c: changing these two includes to absolute paths
  #include "transform360/VideoFrameTransformHandler.h"
  #include "transform360/VideoFrameTransformHelper.h"

• vf_transform360.c: #include "Transform360..." (capital T, as
  suggested in https://github.com/facebook/transform360/issues/24)

• starting over with
  export ELIBS='-lTransform360 -lstdc++'
  export LDFLAGS='-L/usr/local/lib'
  export CFLAGS='-I/usr/local/include/Transform360/Library'
  ./configure --enable-gpl --enable-nonfree --enable-libass --enable-libfdk-aac
  --enable-libfreetype --enable-libmp3lame --enable-libopus
  --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libx264
  --enable-libxvid --enable-libopencv

All of these yielded the same error. Any suggestions would be most welcome. Thank you!