configure.ac should use mysql_config --libs_r instead of manual check for libmysqlclient

[Sergei Glushchenko]

Currently libmysqlclient is checked as following:

AC_CHECK_LIB([mysqlclient], [mysql_init], [DIALOG_LIBS="$DIALOG_LIBS -lmysqlclient"],
    AC_MSG_ERROR(
        [Unable to find libmysqlclient. Please install the mysql development libraries])
)

it should be taken from mysql_config --libs_r similar to include path is taken from mysql_config --include