glGetString(GL_EXTENSIONS) returns no GL_EXT_framebuffer_sRGB, which is required by OpenGL 3.0 (src/mesa/main/version.c compute_version)。so glGetString(GL_VERSION) fall back to OpenGL 2.1。

#include <stdio.h>
#include <stdlib.h>

#define GL_GLEXT_PROTOTYPES
#include <GL/osmesa.h>
//#include <GL/glext.h>

// settings
const unsigned int SCR_WIDTH = 960;
const unsigned int SCR_HEIGHT = 562;

int main(void) {
  OSMesaContext ctx;
  ctx = OSMesaCreateContextExt(OSMESA_RGBA, 16, 0, 0, NULL);
  GLubyte *buffer = malloc(SCR_WIDTH * SCR_HEIGHT * 4 * sizeof(GLubyte));
  OSMesaMakeCurrent(ctx, buffer, GL_UNSIGNED_BYTE, SCR_WIDTH, SCR_HEIGHT);

  printf("GL_RENDERER = %s\n", (char*)glGetString(GL_RENDERER));
  printf("GL_VERSION = %s\n", (char*)glGetString(GL_VERSION));
  printf("GL_VENDOR = %s\n", (char*)glGetString(GL_VENDOR));
  printf("GL_SHADING_LANGUAGE_VERSION = %s\n", (char*)glGetString(GL_SHADING_LANGUAGE_VERSION));
  printf("GL_EXTESIONS = %s\n", (char*)glGetString(GL_EXTENSIONS));

  return 0;
}