SDL_egl: allow creation of versioned OpenGL contexts with EGL_KHR_create_context

If the EGL extension EGL_KHR_create_context is available, we can use it to
set the core/compatability profile and the minimum OpenGL version.

Use this if it is available to get the context requested by the GL attributes.

configure

@@ -21045,6 +21045,7 @@ $as_echo_n "checking for EGL support... " >&6; }
 /* end confdefs.h.  */
 
          #include <EGL/egl.h>
+         #include <EGL/eglext.h>
 
 int
 main ()