This saves a sizeable amount of disk space during the build, by not creating
a very large libjs_static.a (~1.5 GB) that contains a copy of all the symbols
in the object files generated during the build.
Upstream bug: https://bugzilla.mozilla.org/show_bug.cgi?id=2008085
Upstream commit: https://hg.mozilla.org/mozilla-central/rev/a924182c4e55
Note that the upstream commit doesn't apply cleanly to Spidermonkey 128.13.0,
it had to be rebased.
This change reduces the disk space requirement by 50%, from 1.1 GB down to 564 MB.
It is relevant when building 0AD in resource-constrained environments (e.g. in CI).
Olivier Tilloy