Actually I've started running ranlib ../../../binaries/system/*.a as a work around, once I learned of the cause :)
More info: Premake inserts an ar -rcs command into the makefile, which should be doing what we want, according to the ar manpage on Lion:
-c Whenever an archive is created, an informational message to that
effect is written to standard error. If the -c option is speci-
fied, ar creates the archive silently.
-r Replace or add the specified files to the archive. If the ar-
chive does not exist a new archive file is created. Files that
replace existing files do not change the order of the files
within the archive. New files are appended to the archive unless
one of the options -a, -b or -i is specified.
-s Write an object-file index into the archive, or update an exist-
ing one, even if no other change is made to the archive. You may
use this modifier flag either with any operation, or alone. Run-
ning `ar s' on an archive is equivalent to running `ranlib' on
it.
The last sentence is apparently not true, as running ranlib separately on the archive fixes the error.
