Thank you for your tip, I didn't know that you could do a debug session with only one file's symbols.

To coming back to my question (by-passing libinkscape.a creation) there is a log file here (1.4 Mo) of the building:
http://ftpmerou.free.fr/inkscape/Inkscape_build_log.html

I think that it's more a MinGW or Code::Blocks limitation because the returned error is a non sense:
mingw32-g++.exe: C:\devlibs\li: No such file or directory

By surfing, I discovered that ar.exe is an archiver and ".a" just an archive file that can be extracted with a built-in index when ranlib is executed : http://en.wikipedia.org/wiki/Ar_(file_format)

But even when by using some arguments like:

q[f]         - quick append file(s) to the archive
r[ab][f][u]  - replace existing or insert new file(s) into the archive

it just create an new file, copy all data from the original, do its stuff and then deleting the old archive, It takes about the same time... I though that it will just update the current archive.