Summary
When ar jobcache is enabled, emake looks first at the output target name, to see if it matches either .a or .la. Then emake also checks to see if the job is invoking ranlib, libtool or ar, so that it doesn’t accidentally try to apply jobcache to jobs it should not — for example, jobs that happen to generate files named *.a that are not really libraries, or jobs that are not running an archiving tool but instead are doing something like copying a file from one location to another, or extracting the library from a compressed archive, or something else altogether.
Problem
If you are not using one of the above 3 tools to generate static libraries, emake may not detect that ar jobcache should be applied to those jobs.
Solution
There is a simple solution for this problem which involves using an emake pragmafile to explicitly add ar jobcache to these jobs. That could be as trivial as creating a file emake.pragmas with this content:
#pragma jobcache ar %.a:
And invoking emake with --emake-pragmafile=emake.pragmas. This will cause emake to apply ar jobcache to any job with an output matching the pattern %.a, without relying on the heuristics to autodetermine jobcache types.