I tried to install antspyx via pip install antspyx.
Why does pip not found my local ITK installation? It is installed in /usr/lib (package insight-toolkit installed via pacman).
Ok, I managed the No local ITK installation found message by setting the environment variable ITK_DIR=/usr/lib/cmake/ITK-5.2.
Additionally, I had to set the option -DModule_GenericLabelInterpolator:BOOL=ON in the itk pkgbuild file to avoid further issues.