[PATCH] Drop BDB-warning in get-win, if not explicitly building with BDB-support

Hi,

[[[
Drop BDB warnings in gen-make, if building without BDB-support.

* build/generator/gen_win.py
   (__init__): Remove BDB-warning, if optional 'db' library not found in
               self._libraries

* build/generator/gen_win_dependencies.py
   (parse_options): initialize self.bdb_path to None (instead of
'db4-win32')
   (_find_bdb): introduce local variable to determine bdb_path taking
either a
                specified path (via --with-berkeley-db) or attempting the
                default path ('db4-win32')
                Only issue the warning, if failing to locate the BDB
path AND
                the user having explicitly specified the bdb-path.
]]]

Reasoning: BDB support is deprecated and it feels kinda wrong to state
that a deprecated/optional dependency is missing/skipped, when building
subversion.
I kept the old detection behavior (defaulting to db4-win32) however, so
if a user put the bdb-sources in the default path it would find and use
them like with the previous versions (not sure whether u want that
changed or not given the fact that BDB is deprecated).

Regards,
Stefan