The default search pathes for the MySQL library and header files are for hosts
only and do not allow cross-compilation of poco against a target MySQL.
Allow to pass the pathes through the build environment by setting MYSQL_LIBDIR
and MYSQL_INCDIR.
- fixed GH #187: MySQL: allow access to the underlying connection handle
- added GH #186: MySQL: support for MYSQL_SECURE_AUTH
- fixed GH #174: MySQL: 4GB allocated when reading any largetext or
largeblob field