Help with tags

[Glen Stark]

I've been using ggtags with gnu-global for a while now, and been pretty 
happy.  Recently we moved some external dependencies outside of the 
source tree however and I can't figure out how to use them.

Lets say I have two source trees:

/usr/local/external_libs

and

~/my_project

I use emacs on the files in the source tree in my_project, but I'd like 
to be able to follow tags to files in /usr/local/external_libs.

How do I go about doing that?

Thanks very much,

Glen