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 things 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 all relevant artefacts out of spidermonkey's build process.
Depends on D3611 (and until that is merged to svn, this revision will fail to build on the CI).
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.