Install headers required to build external storage plugins

Currently one needs the pre-configured source available to build a storage plugin that's not included in MariaDB. For distribution purposes of those external storage plugins that is not really beneficial, since it most always involves shipping a custom, matching MariaDB package as well.

This can be worked around by installing the necessary headers manually to include them in the MariaDB package (which is what is done in the Slackware package for MySQL and in my SlackBuild script for MariaDB), but that's a distribution specific solution and IMHO would be much better solved upstream.

The corresponding MySQL Bug is: http://bugs.mysql.com/bug.php?id=44722

The approach taken:

• Install some extra headers that are needed for out-of-tree build in
  /usr/include/mysql/private. In .deb put these in a separate package.

• Out-of-tree builder is still responsible for setting same value of options
  like DBUG, SAFEMALLOC, and SAFE_MUTEX.