GLX_*_ARB was not declared in scope
Hi,

I'm having trouble using glad, trying to implement a modern glx context. I've looked at the example given in this repository but keep getting these build errors:

```
-- Configuring done
-- Generating done
-- Build files have been written to: /home/scott/dev/MantaEngine/Build/Engine
-- Configuring done
-- Generating done
-- Build files have been written to: /home/scott/dev/MantaEngine/Build/Editor
[ 37%] Built target GLAD
[ 50%] Building CXX object CMakeFiles/MantaEngine.dir/Source/Core/Linux/X11Window.cpp.o
/home/scott/dev/MantaEngine/Engine/Source/Core/Linux/X11Window.cpp: In function ‘void Manta::WindowCreate(uint32_t, uint32_t, const string&, Manta::Window&)’:
/home/scott/dev/MantaEngine/Engine/Source/Core/Linux/X11Window.cpp:260:25: error: ‘GLX_CONTEXT_MAJOR_VERSION_ARB’ was not declared in this scope
  260 |                         GLX_CONTEXT_MAJOR_VERSION_ARB, 3,
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/scott/dev/MantaEngine/Engine/Source/Core/Linux/X11Window.cpp:261:25: error: ‘GLX_CONTEXT_MINOR_VERSION_ARB’ was not declared in this scope
  261 |                         GLX_CONTEXT_MINOR_VERSION_ARB, 3,
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/scott/dev/MantaEngine/Engine/Source/Core/Linux/X11Window.cpp:262:25: error: ‘GLX_CONTEXT_FLAGS_ARB’ was not declared in this scope; did you mean ‘GL_CONTEXT_FLAGS’?
  262 |                         GLX_CONTEXT_FLAGS_ARB, GLX_CONTEXT_CORE_PROFILE_BIT_ARB,
      |                         ^~~~~~~~~~~~~~~~~~~~~
      |                         GL_CONTEXT_FLAGS
/home/scott/dev/MantaEngine/Engine/Source/Core/Linux/X11Window.cpp:262:48: error: ‘GLX_CONTEXT_CORE_PROFILE_BIT_ARB’ was not declared in this scope; did you mean ‘GL_CONTEXT_CORE_PROFILE_BIT’?
  262 |                         GLX_CONTEXT_FLAGS_ARB, GLX_CONTEXT_CORE_PROFILE_BIT_ARB,
      |                                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                                                GL_CONTEXT_CORE_PROFILE_BIT
/home/scott/dev/MantaEngine/Engine/Source/Core/Linux/X11Window.cpp:278:54: error: ‘glXCreateContextAttribsARB’ was not declared in this scope
  278 |                         x11Data->glGraphicsContext = glXCreateContextAttribsARB(x11Data->display,
      |                                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~
gmake[2]: *** [CMakeFiles/MantaEngine.dir/build.make:104: CMakeFiles/MantaEngine.dir/Source/Core/Linux/X11Window.cpp.o] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:151: CMakeFiles/MantaEngine.dir/all] Error 2
gmake: *** [Makefile:91: all] Error 2
```

Looks like these macros are defined in GL/glxext.h which I haven't found to be defined anywhere within glad. Any help would be greatly appreciated :)