I tried to reproduce a similar set of directories as yours. I couldn't 
get the "circular dependency" error, but I had to compile everything in 
one command line, otherwise the file a.cma (which is in the _build/A 
directory) was deleted before compiling b.cma. The command which I used 
and which worked is:

ocamlbuild -Is A,B a.cma b.cma

I don't really understand how the -lib option works though. Where and 
when does it look for the library x.cma with the "-lib x" option? How 
and what's the semantic of the "+" you can add at the beginning of a 
library name?...

-- 
Romain Bardou