Most OpenGL tutorials tell you to load OpenGL functions either by using some form of *GetProcAddress (even SDL2 provides one of those, use it if you want proper multi-platform support without GLEW), or by using some library such as GLEW or GLFW.
On most UNIXlike systems, basic dynamic linking with the libGL.so* library works just fine. Even with NVidia. This means that you can straight up use all functions your driver supports, just by adding `pkg-config --libs gl` to your LDLIBS.
Headers you can get by #include GL/gl.h, and GL/glext.h, with
-DGL_GLEXT_PROTOTYPES in your CFLAGS.
If you want to use OpenGL ES, use it's headers (GLES2/gl2.h) and libraries
(-lGLESv2) instead, and set SDL GL attribute SDL_GL_CONTEXT_PROFILE_MASK to
SDL_GL_CONTEXT_PROFILE_ES. Or better yet, use pkg-config (--libs/--cflags) glesv2.