[Mead] MEAD on MacOS

[Alex Breuer]

I've managed an apparently successful MEAD install on MacOS 10.3; the
only problem is that in OS 10, Berkeley DB's only consist of one file,
and that ends with extension '.db'.  Appropriate adjustments to the
Essence::IDF module correct the problem (simply replace the test which
determines the existence of the idf database to read 'filename.db'
instead of 'filename.dir').  Also, the auto-install of the required
Perl modules did not work, I manually installed mine; I haven't tried
to mess with Install.PL.  So far tests with the data included in the
MEAD distribution match results from MEAD on Solaris.

-AB