Compiling ffmpeg with alpha channel support in libx265

I need to encode a transparent video in the H.265/HEVC format. Recently, the x265 software encoder library used by ffmpeg has added support for the alpha channel (docs).
I compiled ffmpeg as per the instructions from here with the required ENABLE_ALPHA option set during build of libx265.
However, when I execute ffmpeg -h encoder=libx265, there are no supported pixel formats with the alpha channel:

Encoder libx265 [libx265 H.265 / HEVC]:
General capabilities: dr1 delay threads
Threading capabilities: other
Supported pixel formats: yuv420p yuvj420p yuv422p yuvj422p yuv444p yuvj444p gbrp gray

It’s the same when I do ffmpeg -h encoder=libx265 -x265-params alpha=1.

Do I need to somehow explicitly tell ffmpeg that the library supports new pixel formats?