Header glx/glxext.h prevents the use of GLX_CONTEXT_RELEASE_BEHAVIOR_ARB

I am trying to setup a OpenGL GLX context using the library GLFW and using it against a Xvfb virtual framebuffer (I am using xserver 1.19.6 obtained from Ubuntu package xserver-xorg-core)

When I create the context I need to specify specific window hints as follows

void error_callback(int, const char* err_str)
{
    std::cout << "GLFW Error: " << err_str << std::endl;
}


int main(int argc, char** argv)
{

    glfwSetErrorCallback(error_callback);
    /* Initialize GLFW. */
    if (glfwInit() == GL_FALSE)
    {
        std::cerr << "Failed to initialize GLFW." << std::endl;
        return false;
    }


    /* Set context properties by "hinting" specific (property, value) pairs. */
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR,    3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR,    3);
    glfwWindowHint(GLFW_OPENGL_PROFILE,           GLFW_OPENGL_CORE_PROFILE);
    glfwWindowHint(GLFW_RESIZABLE,                GL_FALSE);
    glfwWindowHint(GLFW_VISIBLE,                  GLFW_FALSE);
    glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT,    GL_TRUE);
    glfwWindowHint(GLFW_CONTEXT_RELEASE_BEHAVIOR, GLFW_RELEASE_BEHAVIOR_NONE);


    /* Create window to create context and enquire OpenGL for the maximum size of the renderbuffer */
    GLFWwindow* window_ = glfwCreateWindow(1, 1, "OpenGL window", nullptr, nullptr);
    if (window_ == nullptr)
    {
        std::cerr << "Failed to create GLFW window." << std::endl;;
        glfwTerminate();
        return false;
    }

    return  EXIT_SUCCESS;
}

However, using the hint glfwWindowHint(GLFW_CONTEXT_RELEASE_BEHAVIOR, GLFW_RELEASE_BEHAVIOR_NONE); causes the application to fail with the following error

Error: GLX: Failed to create context: BadValue (integer parameter out of range for operation).

In particular this problem is happening due to the following lines of codes in glx/createcontext.c

#ifdef GLX_CONTEXT_RELEASE_BEHAVIOR_ARB
        case GLX_CONTEXT_RELEASE_BEHAVIOR_ARB:
            flush = attribs[2 * i + 1];
            if (flush != GLX_CONTEXT_RELEASE_BEHAVIOR_NONE_ARB
                && flush != GLX_CONTEXT_RELEASE_BEHAVIOR_FLUSH_ARB)
                return BadValue;
            break;
#endif

        default:
            return BadValue;
        }
    }

where the part of code between #ifdef GLX_CONTEXT_RELEASE_BEHAVIOR_ARB and #endif is never compiled and the default return BadValue is fired. Compilation does not happen since GLX_CONTEXT_RELEASE_BEHAVIOR_ARB is defined in <GL/glxext.h> but this header is not included as can be seen in glx/glxext.h

/* doing #include <GL/glx.h> & #include <GL/glxext.h> could cause problems
 * with overlapping definitions, so let's use the easy way
 */
#ifndef GLX_RGBA_FLOAT_BIT_ARB
#define GLX_RGBA_FLOAT_BIT_ARB             0x00000004
#endif
#ifndef GLX_RGBA_FLOAT_TYPE_ARB
#define GLX_RGBA_FLOAT_TYPE_ARB            0x20B9
#endif
#ifndef GLX_RGBA_UNSIGNED_FLOAT_BIT_EXT
#define GLX_RGBA_UNSIGNED_FLOAT_BIT_EXT    0x00000008
#endif
#ifndef GLX_RGBA_UNSIGNED_FLOAT_TYPE_EXT
#define GLX_RGBA_UNSIGNED_FLOAT_TYPE_EXT   0x20B1
#endif

Unfortunately, by not including <GL/glxext.h> the defines GLX_CONTEXT_RELEASE_BEHAVIOR_ARB, GLX_CONTEXT_RELEASE_BEHAVIOR_NONE_ARB and GLX_CONTEXT_RELEASE_BEHAVIOR_FLUSH_ARB are not present causing the issue described above.

Is there any specific reason behind this choice?