GLFW on Linux only needs to be linked with libGL since version 3.1.2
* **I'm submitting a ...**
  - [x] bug report
  - [ ] feature request

* **What is the current behavior?**

NimGL links all these unnecessary libraries when compiling GLFW:

https://github.com/nimgl/nimgl/blob/2ffe9e1d527aab6085decfbb56741276c480a5df/src/nimgl/glfw.nim#L52

Excerpt from the GLFW changelog:

![image](https://user-images.githubusercontent.com/16415678/96040863-3cd7d780-0e6b-11eb-8b63-6e724edf2ae4.png)


* **If the current behavior is a bug, please provide the steps to reproduce and
if possible a minimal demo of the problem**

Just use NimGL's GLFW wrapper.

* **What is the expected behavior?**

NimGL should only link to libGL and *maybe* pthread.

* **What is the motivation / use case for changing the behavior?**

By linking to all the X11-related libraries we're only pulling in unnecessary *hard* dependencies to executables. This might prove running NimGL executables difficult on Wayland-only environments. Also, requiring users to install all of those libraries is just plain inconvenient in the long run. 

* **Please tell us about your environment:**
  
  - Version: 1.1.5
  - OS: Linux
  - Bindings: GLFW


* **Other information** (e.g. detailed explanation, stacktraces, related issues,
suggestions how to fix, links for us to have context, eg. stackoverflow,
docs.gl, documentation etc)

The solution should be simple; replace the current line with this:
```nim
{.passL: "-pthread -lGL".} 
```
By the way, linking to libm is unnecessary as Nim links it by default.