Build MMK message file in source directory.

The first part of the changes to include mmk_msg.h as part of the source
repository.  The file is now placed in the source directory, after SDL
has built it, rather than ETC_DIR.

This change references #59.
MMK message now in source.

There are the message definitions, now stored in the repository.

This fixes #59.
Merge pull request #61 from endlesssoftware/issue-59

Move MMK_MSG.H into source...