build: Fix warnings in auxiliary files

build-support/compiler-workarounds/ostream-operator-nullptr.h

@@ -7,7 +7,7 @@
 namespace std
 {
 inline std::ostream&
-operator<<(std::ostream& os, std::nullptr_t ptr)
+operator<<(std::ostream& os, std::nullptr_t)
 {
     return os << "nullptr"; // whatever you want nullptr to show up as in the console
 }

build-support/empty-main.cc

@@ -1,4 +1,5 @@
 int
 main()
 {
+    return 0;
 }