feat: add a feature flag to build from source

[wusyong]

This is an feature request starting from zulip: https://servo.zulipchat.com/#narrow/stream/263398-general/topic/Spidermonkey.20dylib.20in.20mozjs/near/406266292
Basically, I hope we can offer a feature flag to toggle building mozjs's spidermonkey from source.
Because 99% of people always stuck on building mozjs when they want to try servo.
It would be awesome if beginners can turn the flag off and download mozjs's spidermonkey binary without compiling and bindgen.
rusty_v8 already has mechanism to do so.

[wusyong]

Currently I'm testing linking existing static libraries and including generated rust files. It seems working so far.
The static libs need to be linked are: libjs_static.a, mozjs-sys's libjsglue.a, and mozjs's libjsglue.a.
And the generated rust files are: jsapi.rs and gluebindings.rs.
I'm not sure where to put the files yet. But if there's no suggestion, I'll start by putting them in OUT_DIR without any additional directory path.
And I still hope we can push #409, so there will be less files to worry about.

[sagudev]

Great stuff, we would probably need two variants one with debug-mozjs feature and one without. Although we could just make release version and if debug-mozjs feature is activated we build it from source.

[sagudev]

We would need to extend CI, to automagically produce prebuilds and then make tags for every commit (given that mozjs is not versioned), although this could result a lot of tags.

[wusyong]

Great stuff, we would probably need two variants one with debug-mozjs feature and one without. Although we could just make release version and if debug-mozjs feature is activated we build it from source.

I guess one release version (with streams feature) is good enough for now. We don't have enough resources to support multiple version at the moment.

We would need to extend CI, to automagically produce prebuilds and then make tags for every commit (given that mozjs is not versioned), although this could result a lot of tags.

Maybe we can produce prebuilds on version release. It's also a good improvement for servo since it's still using git dependency I believe. prebuilds per version release seems pretty reasonable and should be more stable for users.

[wusyong]

Closed by #450
@sagudev Thanks for reviewing!

[sagudev]

Thank you for all hard work.

I will update companion PR later today.