OK, I use Homebrew and I wanted to get Hangouts to work for Bitlbee.
First, install Pidgin:
brew install pidgin
Build Bitlbee with libpurple support:
./configure --purple=1 make make install
(I use ./configure --debug=1 --asan=1 --purple=1 because I'm still working on Mastodon support.)
If you run it, you should see a lot more protocols -- but still no Hangouts.
Install Mercury and Protocol Buffers implementation in C in order to build the Hangouts Plugin for libpurple. You'll note that there are also other dependencies but I already had all the other stuff installed. Nothing tricky to figure out for yourself, I hope. If you run into problems, leave a comment and I'll see whether I can figure it out.
brew install hg brew install protobuf-c hg clone https://bitbucket.org/EionRobb/purple-hangouts cd putple-hangouts make make install
When you start Bitlbee, make sure you use -F instead of -D or you'll get an error as soon as you try to sign into an account using libpurple.
If you restart Bitblee now, the plugins command should list hangouts.
Set it up:
account add hangouts <your email address> account on
You'll be contacted by a fake account called purple_request with instructions on how to get the OAuth token. It will link you to a YouTube video but the description of the video are the real instructions.
Answer purple_request with the token and you should be up and running.