Re-generate srtc/gl/lib.rs whenever `make lib` is called

We now depend on https://github.com/bjz/khronos-api as a submodule. This is much more robust than relying on the [Kronos API repository](https://cvs.khronos.org/svn/repos/ogl/trunk/doc/registry/public/api/), which funnily enough seems to be 404-ing at the time or writing.

The following variables can be customised when compiling the library:

~~~make
GL_NAMESPACE        ?= gl
GL_API              ?= gl
GL_PROFILE          ?= core
GL_VERSION          ?= 4.3
GL_EXTENSIONS       ?=
GL_FULL             ?=
GL_XML              ?= $(DEPS_DIR)/khronos-api/$(GL_API).xml
~~~

For example:

~~~
GL_VERSION=2.1 GL_PROFILE=compatability make lib
~~~