いろんな所から拝借して組み合わせたコードです。

opengl_glfw_test.cpp

#define GLFW_INCLUDE_GLU
#include <gl/glfw3.h>
#include <iostream>
#include <cstdlib>
#include <string>

int main()
{
  GLFWwindow* window;

  if (!glfwInit()) return -1;

  window = glfwCreateWindow(640, 480, "Hello OpenGL", nullptr, nullptr);
  if (!window) {
    glfwTerminate();
    return -1;
  }

  glfwMakeContextCurrent(window);
  while (!glfwWindowShouldClose(window)) {

    // OpenGLでの描画処理をここに書く

	float ratio;
        int width, height;
        glfwGetFramebufferSize(window, &width, &height);
        ratio = width / (float) height;
        glViewport(0, 0, width, height);
        glClear(GL_COLOR_BUFFER_BIT);
        glMatrixMode(GL_PROJECTION);
        glLoadIdentity();
        glOrtho(-ratio, ratio, -1.f, 1.f, 1.f, -1.f);
        glMatrixMode(GL_MODELVIEW);
        glLoadIdentity();
        glRotatef((float) glfwGetTime() * 100.f, 0.f, 0.f, 1.f);
        glBegin(GL_TRIANGLES);
        glColor3f(1.f, 0.f, 0.f);
        glVertex3f(-0.6f, -0.4f, 0.f);
        glColor3f(0.f, 1.f, 0.f);
        glVertex3f(0.6f, -0.4f, 0.f);
        glColor3f(0.f, 0.f, 1.f);
        glVertex3f(0.f, 0.6f, 0.f);
        glEnd();

    glfwSwapBuffers(window);
    glfwPollEvents();
  }
  
  glfwTerminate();
  
  return 0;
}