gl-renderer: Only check for subimage when we have extension at compile time

If weston is compiled against a gl2ext.h that doesn't have the subimage
extension, but then run against a gles2 library that does provide it,
we end up disabling the glTexImage2D falback without having the subimage
code paths compiled in.

src/gl-renderer.c

@@ -2085,8 +2085,10 @@ gl_renderer_setup(struct weston_compositor *ec, EGLSurface egl_surface)
 	else
 		ec->read_format = PIXMAN_a8b8g8r8;
 
+#ifdef GL_EXT_unpack_subimage
 	if (strstr(extensions, "GL_EXT_unpack_subimage"))
 		gr->has_unpack_subimage = 1;
+#endif
 
 	if (strstr(extensions, "GL_OES_EGL_image_external"))
 		gr->has_egl_image_external = 1;