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.
The version-tracked Windows build artefacts are moved to `libraries/win32/` so as to keep them out the way (if this revision gets accepted, the move will be committed separately).
Depends on D3611 (but contains the changes contained within that revision, as the CI is not smart enough to apply it first)