Installing *messages.h files doesn't seem to be trivial

For Kea 1.5.0 beta2 we attempted to install all *_messages.h files which contain labels of log messages used by loggers. That turned out to be a problem for make distcheck because it requires to compile .mes files using the message compiler from the $(top_builddir) where the kea-msg-compiler is not available. At the stage where the compiler is needed the compiler is presumably available in the $(top_distdir) instead. We haven't figured out why this issue occurs when we attempt to install the files and not when we don't. Because this issue was found right before the Kea 1.5.0 beta2 release we didn't have time to investigate it and find the proper solution. We simply backed off the changes (we don't install message headers) hoping for some solution for it later.

We should also consider whether the messages files should be installed at all. It seems they will only be needed if there are any header files installed which require the message files. That could be the case if we have a template class implemented within the header file which requires logging. As far as we can tell today, we don't have such cases in the code. So, backing off the changes seemed safe for Kea 1.5.0 beta2.