Using malloc-lib=jemalloc in MariaDB 10.2 causes non-critical error about missing mysql_config on startup

When using a custom memory allocator, e.g.

[mysqld_safe]

malloc-lib=jemalloc

When starting MariaDB, the mysqld_safe script attempts to learn which lib directories to use by executing mysql_config command.

However this is only provided by the MariaDB-devel package, and so isn't installed on most production servers.

As such, we get an error each time we start MariaDB, in the form of:

180703 12:52:39 mysqld_safe Can not run mysql_config --variable=pkglibdir from '/usr/bin/mysql_config'

MariaDB still starts OK because the mysqld_safe script also looks in the normal lib directories, where jemalloc is installed to by CentOS.

I do not think we need to see this error, as it will be common for mysql_config not to be installed. Perhaps only show the error if the allocator library isn't found in any of the normal library directories?