Add/Fix support for non-bundled libedit in client

In the mysql.cc client source file, MariaDB uses declaration of functions from the readline library (such as rl_library_version or history_length). This however makes it impossible to link with non-bundled version of the libedit library - the functions are simply not there and the linking fails with undefined reference errors.

Also the CMake gives no options for finding and linking the non-bundled library.

As the Oracle's MySQL includes support for non-bundled libedit, it would be nice if MariaDB could provide this capability too.