Adapts name of the mysql client library

[stefanproell]

Prior to MySQL 5.5 there used to be two versions of libmysqlclient. The version with the _r suffix was threadsafe, the one without was not. Since the release of MySQL 5.5, both versions are threadsafe and the _r version is only a symlink to libmysqlclient.so.

I quote the MySQL 5.5 Release Notes below:

The libmysqlclient client library is now built as a thread-safe library. The libmysqlclient_r client library is still present for compatibility, but is just a symbolic link to libmysqlclient.

In Ubuntu (and probably other Debian based systems), the libmysqlclient_r.so was removed and so was the symlink. Thus the compilation fails with the following error:

CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
Please set them or make sure they are set and tested correctly in the CMake files:
MYSQL_LIB
    linked by target "mysql_client" in directory /home/sproell/git/query-playback/percona_playback/mysql_client
 
-- Configuring incomplete, errors occurred!
See also "/home/sproell/git/query-playback/build_dir/CMakeFiles/CMakeOutput.log".
See also "/home/sproell/git/query-playback/build_dir/CMakeFiles/CMakeError.log".

As a fix for this issue, I recommend replacing libmysqlclient_r with libmysqlclient to be compatible with more recent releases.