support android build and glue

[aydinkim]

Fixed according to your advice.
Is it fine?

As Ilyong's comment, actually It is needed android-glue in android.
we need to separate the each platforms in makefile.in?
we added linking libandroid-glut.a on making librustglut.so.
It will be fine at linux or macos build.
however it seems not obious for other people so that it may causes some confusion.
I don't know what is better. shorter code is better or clear code is better..

Let me know your opinion, and we will be very appreciated.

[metajack]

This isn't pretty, but will have to do for now.

A better fix @brson suggests is to refactor stuff so that libservo doesn't talk to the windowing system. This is going to be required anyway for embedding, so this android-glue hack will eventually go away.

[aydinkim]

@metajack We are agreed to your opinion eventually. The reason of using android version of freeglut is servo originally used freeglut. But there is some differences between them actually.
And currently servo's window system has changed to glfw, unfortunately, glfw doesn't support android port yet.
We are now considering this issue and it can be resolved step by step.