I added this like immediately below the
glBindBuffer(GL_ARRAY_BUFFER, vbo_triangle);

but it doesn't work either? where do I place it.

I'm now starting to learn Opengl tutorials from Wikibooks[http://en.wikibooks.org/wiki/OpenGL_Programming]. In the tutorial...

there will be no folders named "mesa". look for "GL" in /usr/include/
then probably you got opengl installed in your system