Taken from: https://github.com/dlundquist/sniproxy/commit/822bb80df9b7b345cc9eba55df74a07b498819ba
Author: Pierre-Olivier Mercier <nemunaire@nemunai.re>

--- a/src/http.h
+++ b/src/http.h
@@ -29,6 +29,6 @@
 #include <stdio.h>
 #include "protocol.h"
 
-const struct Protocol *const http_protocol;
+extern const struct Protocol *const http_protocol;
 
 #endif
--- a/src/tls.h
+++ b/src/tls.h
@@ -28,6 +28,6 @@
 
 #include "protocol.h"
 
-const struct Protocol *const tls_protocol;
+extern const struct Protocol *const tls_protocol;
 
 #endif