Flawed ports's build process created the library that is linked to non-existent dynamic library:

ldd -a work/stage//usr/local/lib/lv2/convo.lv2/convoLV2.so

work/stage//usr/local/lib/lv2/convo.lv2/convoLV2.so:
libzita-convolver.so.3 => not found (0)

Currently, proxydeps misses such situation. I made it to print an error message in such case.