[#1743] fix circular dependency in src/lib/log

[#1743] 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.