Adopt __printflike macro for -Wformat-nonliteral

lib/nghttp2_helper.h

@@ -35,6 +35,10 @@
 #include <nghttp2/nghttp2.h>
 #include "nghttp2_mem.h"
 
+#ifndef __printflike
+#  define __printflike(fmtarg, firstvararg)
+#endif
+
 #define nghttp2_min(A, B) ((A) < (B) ? (A) : (B))
 #define nghttp2_max(A, B) ((A) > (B) ? (A) : (B))
 

lib/nghttp2_session.c

@@ -147,6 +147,7 @@ static int check_ext_type_set(const uint8_t *ext_types, uint8_t type) {
   return (ext_types[type / 8] & (1 << (type & 0x7))) > 0;
 }
 
+__printflike(3, 4)
 static int session_call_error_callback(nghttp2_session *session,
                                        int lib_error_code, const char *fmt,
                                        ...) {