I've got some code in lp:drizzle/staging right now that's on its way (barring major catastrophes) to trunk. It's not code that does anything sexy as far as the actual running server is concerned. It's a code cleanup branch.
Anyway - short story being - everything from mysys and mystrings that is actually part of public APIs has been moved into drizzled/ proper. Everything else has been moved into drizzled/internal. None of the headers from drizzled/internal are installed... so none of the headers in drizzled/ should be using any of them. Combine this with the past week's removal of both server_includes.h and global.h, and we're getting pretty close to having fully consumable headers.
Which brings me to:
In doing this, I noticed a bunch of things that either need to be fixed, still need to be deleted, or need to be put behind a namespace so that including our headers doesn't strangely and unexpectedly b0rk someone's application. To that end, I've added a bunch of new low-hanging-fruit blueprints. As always, you can look at the full list on Launchpad but I'm including links to the new ones here.
New Drizzle Low Hanging Fruit: