package: add the poco C++ libraries collection

Add the C++ Portable Components libraries.

The Data/ODBC components is disabled because its dependency (unixODBC) is not
in Buildroot.

The Data/MySQL component is build tested only. It probably requires an
additional RPATH to actually run, because libmysqlclient resides in a
non-standard location under /usr/lib/mysql.

[Peter: use depends on for toolchain options]
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>