Typically used with umbrella headers, e.g. GTK:
#if !defined (GTK_H_INSIDE) && !defined (GTK_COMPILATION)
#error "Only <gtk/gtk.h> can be included directly."
Heuristic is fairly conservative, a quick code search over github showed
a fair number of hits and few/no false positives. (Not all were umbrella
headers, but I'd be happy avoiding include insertion for all of them).
We may want to relax the heuristic later to catch more cases.