By default when creating a library file, if the file does not exist, ar issues a warning to the error output.
This can be prevented by using the c option.
See http://unixhelp.ed.ac.uk/CGI/man-cgi?ar for more details.
This warning is really annoying for me because it yields to an error being logged by cruisecontrol...
Attached you can find a (very very simple) patch to correct this issue : it simply adds the c option to the ar command line.