add DBD::mysql::client_version() to have the client version without DB connections

[gfx]

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The value of mysql_get_client_version() is available via $dbh->{clientversion}, but it would be nicer if we can get the client library version without DB connections; For example, one would like to add or remove mysql_skip_secure_auth=1 to the dsn according to the client library version.

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(edited)

As @miyagawa said, the following trick works as well:

$ perl -E 'use DBI; say(DBI::_new_dbh(DBI->driver("dbi:mysql:"))->{mysql_clientversion});'
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