libtool adding extra 'U' to .so file names

I have a project built with autotools, divided into two folders; the first one, lib, creates some libraries, and the second one, tools, links against them.
The build fails because libtool renames the libraries as follows while relinking:

mylib.1.0.0 -> mylib.1.0.0U

After the renaming occurs, anyway, the previous name is not restored, nor is a new mylib.1.0.0 created, so the symbolic links like mylib still point to the name without the extra U and the linking fails.
Is there any way to avoid this? or maybe even to avoid the relinking at all? I'm using libtool version 1.5.6.

Solution

Apparently I got it...
The problem is easily reproducible with the following structure:

a lib folder, building:
- liba
- libb, depending on liba
a src folder, building prog, which links libb

Then you must specify this in lib/Makefile.am:

lib_LTLIBRARIES = \
    libb.la \
    liba.la

At that point a liba.so.1.0.0U file will be produced. Specifying instead (note that the order of the libraries is changed)

lib_LTLIBRARIES = \
    liba.la \
    libb.la

the build works fine.
I agree that specifying things in the correct order is in any case the best thing to do; what is still unclear to me is if this is a libtool bug or not...