[Bro-Dev] Move 3rdparty into a separate submodule

[Bernhard Amann]

Hi,

I just wanted to ask what you think about moving the 3rdparty stuff under src (which at
the moment only contains sqlite) into a separate git submodule?

That way we keep the 3rdparty sources and their change history out of Bro history. SQLite 
alone already has more than 140k lines - and I kind of suspect that we might other 3rdparty 
sources in the future.

The disadvantage of this approach is, that the bro git repository does no longer contain
all source-files necessary to compile bro. On the other hand - stuff like cmake already is
in submodules - so even though the bro git contains all source files at the moment, you
still cannot compile it without checking out the submodules.

Bernhard