protoc-gen-swipl should be in same directory as swipl executable

[kamahen]

Currently, to run protoc for Prolog, the directory /usr/lib/swi-prolog/library/protobufs needs to be in the $PATH. It would be better if protoc-gen-swipl were in the same directory as the swipl executable (e.g., /usr/lib/swipl).

This could be done with a symlink from /usr/lib/protoc-gen-swipl to /usr/lib/swi-prolog/library/protobufs/protoc-gen-swipl -- how would this be specified in protobufs/CMakeLists.txt?