databases/mysql++3: Fix build with MariaDB 10.2

./lib/dbdriver.cpp:260:10: error: use of undeclared identifier 'CLIENT_LONG_PASSWORD'

(o >= CLIENT_LONG_PASSWORD) &&
      ^

1 warning and 1 error generated.
gmake[1]: *** [Makefile:545: mysqlpp_dbdriver.o] Error 1

PR: 229950
Submitted by: Ting-Wei Lan <lantw44@gmail.com> (maintainer)