Add link to gl library if necessary

[sotaroikeda]

This fixes issue #84

Added link to gl library on linux and windows.

[sotaroikeda]

Add links to a library that is used by StaticGenerator

[emilio]

Why doesn't this link to the GL framework in OSX or EGL in android?

[sotaroikeda]

I did not add it because StaticGenerator is not used on mac. But instead, cgl is used and cgl defines functions of GL framework :(

It seems better to revert linking to gleam, since gleam is used out side of offscreen_gl_context. Another crates expects the library is linked by gleam.

gleam<-cgl<-glutin
<-offscreen_gl_context
<-winit

[sotaroikeda]

I did not add EGL in android since the link was already requested by the following.
https://github.com/emilio/rust-offscreen-rendering-context/blob/master/build.rs#L22

[sotaroikeda]

I chose to add link at offscreen_gl_context. I updated a patch as to add link on mac. On android case, link to EGL is already exist in build.rs.

[sotaroikeda]

@emilio, can you review again? Thanks.

[emilio]

r=me, thanks!

[glennw]

@sotaroikeda @emilio I'm working on the WR update in Servo - but getting a link error on android due to it trying to link libGL - any ideas where this might be coming from? The log is http://build.servo.org/builders/android/builds/5902/steps/compile/logs/stdio from servo/servo#16073 (comment).

[sotaroikeda]

It seems to be addressed now by 7e00056