It is important to be careful when using parallel execution (the
-j switch; see Parallel Execution) and archives.
If multiple ar commands run at the same time on the same archive
file, they will not know about each other and can corrupt the file.
Possibly a future version of make will provide a mechanism to
circumvent this problem by serializing all commands that operate on the
same archive file. But for the time being, you must either write your
makefiles to avoid this problem in some other way, or not use -j.