As mentioned in D3611, we currently only use `pkg-config` with `spidermonkey` when the `--with-system-mozjs` argument is passed to `update-workspaces.sh`.
This revision changes that, so we //always// use `pkg-config` with `spidermonkey` on Linux, BSD, and Mac OSX systems. In the process, this revision also changes the `spidermonkey` build process to use `make install` instead of manually copying relevant artefacts out of `spidermonkey`'s build process.
This is (a) a test to see if it passes CI, and (b) a test to see if this change is desired.
A previous version of this revision moved the version-tracked Windows build artefacts to `libraries/win32/` so as to keep them out the way. This is no longer being done (as it made this too large to be reviewable) but could be reconsidered.
Depends on D3611 (but contains the changes contained within that revision, as the CI is not smart enough to apply it first)