Convert list to string for COMMENT argument

[stima]

The issue occurs because CMake expects a single string for the COMMENT argument, but when a list is passed, it tries to expand the list into multiple arguments. To resolve this convert lists to strings using string(REPLACE ...) to join list items into a single string.

CMake Warning (dev) at <path>/protobuf-generate.cmake:150 (add_custom_command):
  COMMENT requires exactly one argument, but multiple values or COMMENT
  keywords have been given.