Resolve "circular dependency in src/lib/log"

Closes #1743 (closed).

• ced449d6 fix circular dependency in src/lib/log

  Fixes the problem where to generate log_messages.(cc|h), you need kea-msg-compiler, but to compile kea-msg-compiler you need log_messages.(cc|h). This halted compilation even when building from scratch.

  Simply only regenerate log_messages.(cc|h) only if kea-msg-compiler exists. It will not regenerate the files on first pass, but one can explicitly call make messages -C src/lib/log afterwards to regenerate the files.