Looks like commit revno 3363 for bug lp:886550 broke builds for clients.
How to repeat:
1) Grab a fresh bzr 5.5 tree
2) cmake. -DCMAKE_INSTALL_PREFIX=/usr/local/mariadb-5.5
3) make; make install
When trying to compile a client (in this case myodbc5) it fails with an error in m_string.c (204): #include <mysql/plugin.h>.
The include/mysql/mysql subdirectory does not exist anymore