Thanks a lot Pierrick, now I have a set of working makefiles:

# this is Makefile.a

SOURCES = a.ml
RESULT = a
all: byte-code-library
-include OCamlMakefile

# this is Makefile.b

SOURCES = b.ml
RESULT = b
all: byte-code-library
-include OCamlMakefile

# this is Makefile.test

SOURCES = test.ml
LIBS = str a b
RESULT = test
all: byte-code
-include OCamlMakefile


Le 06/03/2015 11:44, Pierrick Couderc a écrit :
> Hi,
 >
 > I'm not an expert at all with OCamlMakefile, but I guess that comes 
from the fact the compiler embeds Str in both libraries "a.cma" and 
"b.cma". Doing it by hand:
 >
 > $ ocamlc -a -o a.cma a.ml
 > $ ocamlc -a -o b.cma b.ml
 > $ ocamlc -o a.out str.cma a.cma b.cma test.ml
 >
 > You don't need to give "str" as Lib when generating your libraries, 
only when producing your executable :-) If you give str.cma when 
generating your libraries:
 >
 > $ ocamlc -a -o a.cma str.cma a.ml
 > $ ocamlc -a -o b.cma str.cma b.ml
 > $ ocamlc -o a.out a.cma b.cma test.ml
 >   File "b.cma(Str)", line 1:
 >   Warning 31: files b.cma(Str) and a.cma(Str) both define a module 
named Str
 >
 >
 > Pierrick
 >
 > 2015-03-06 11:16 GMT+01:00 Vu Ngoc San <san.vu-ngoc@laposte.net>:
 >
 >     Hello list
 >
 >     I have a basic compilation question:
 >     I want to write two independent libraries, say A an B, and both 
use a third library, eg. Str
 >
 >     When I write a program which uses A and B, I get
 >     File "b.cma(Str)", line 1:
 >     Warning 31: files b.cma(Str) and a.cma(Str) both define a module 
named Str
 >
 >     How to get rid of this warning ? Am I doing something wrong ?
 >
 >     Thanks
 >     San
 >
 >     ----
 >
 >     Here is a minimal example:
 >
 >     (* This is a.ml *)
 >
 >     let version = "A";;
 >     let _ = print_endline (Str.string_before "Loading the A library 
-- foo" 22);;
 >
 >     (* This is b.ml *)
 >
 >     let version = "B";;
 >     let _ =  print_endline (Str.string_before ("Loading the " ^ 
version ^" library -- bar") 23);;
 >
 >     # this is Makefile.a
 >
 >     SOURCES = a.ml
 >     LIBS = str
 >     RESULT = a
 >     all: byte-code-library
 >     -include OCamlMakefile
 >
 >     # this is Makefile.b
 >
 >     SOURCES = b.ml
 >     LIBS = str
 >     RESULT = b
 >     all: byte-code-library
 >     -include OCamlMakefile
 >
 >     (* this is test.ml *)
 >
 >     let _ = print_endline (Str.string_before ("Hello " ^ A.version ^ 
" and " ^ B.version ^"---")  13);;
 >
 >     # this is Makefile.test
 >
 >     SOURCES = test.ml
 >     LIBS = a b
 >     RESULT = test
 >     all: byte-code
 >     -include OCamlMakefile
 >
 >
 >
 >     --
 >     Caml-list mailing list.  Subscription management and archives:
 >     https://sympa.inria.fr/sympa/arc/caml-list
 >     Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
 >     Bug reports: http://caml.inria.fr/bin/caml-bugs
 >
 >
 >
 >
 > --
 > Pierrick COUDERC
 >
 > PhD Student at OCamlPro / ENSTA Paristech Université Paris Saclay