The build configuration contains [two sets of header checks](https://gitweb.torproject.org/tor.git/tree/configure.ac#n945) where one checks headers that are deemed essential and one checks headers that are not. The only difference being that missing essential headers display a non-fatal warning when they aren't found. For the following reasons i think it is better to merge the sets and discard the warning. - Commit [e8cc839e41adc4975a61fee62abe7f6664fd0c0e](https://gitweb.torproject.org/tor.git/commit/?id=e8cc839e41adc4975a61fee62abe7f6664fd0c0e) adds `sys/capability.h` to the essential header set but Tor runs fine without capabilities being available. IMO it is in the wrong set. - Having two sets requires developers to make a choice whether headers are essential or not. Instead developers should write code that handles cases where the headers are unavailable (like the capabilities code). - The current warning is truncated because its message is incorrectly quoted. This means the part about sending the `orconfig.h` file isn't shown and has been ineffective.