support android build

[aydinkim]

Support android build
This commit does not affect to each submodule's original standalone build system.
Added android freeglut binder.

[metajack]

Just add this cfg attribute to the linux section previously.

[aydinkim]

Ok, we will integrate linux and android block.

[metajack]

I do not understand what android-glue is doing. Can you explain what is going on here?

[ILyoan]

This version of servo-android port is using libfreeglut 3.0 which supports android.(I think we should transfer to glfw right?)
Unfortunately, the libfreeglut has android entry and main event loop in it and thus it should be statically linked to user application so that android system could find android main entry.
The problem in this situcation is that the servo engine could not link libfreeglut directly because the servo android application(libfreeglut statcally linked with it) dynamically links servo engine(libservo.so).
The purpose of android-glue is to provide servo engine with a path to call libfreeglut functions. Servo app would register callbacks for each glut function before starts servo engine. and then, when the callback was invoked by servo engline, the callback function would actually call the mapped glut function.

This was the best option for me when I was working on servo android.
If you have better soultion or advice, it would be very appreciated.

[aydinkim]

Adapted another PR. this commit is invalid. Close.