Fix build on macOS 10.14.2 (#1509)

wpilibc/src/main/native/include/frc/PIDSource.h

@@ -19,6 +19,8 @@ enum class PIDSourceType { kDisplacement, kRate };
  */
 class PIDSource {
  public:
+  virtual ~PIDSource() = default;
+
   /**
    * Set which parameter you are using as a process control variable.
    *

wpiutil/src/test/native/cpp/test_optional.cpp

@@ -625,6 +625,9 @@ TEST(Optional, example_rationale)
   optional<optional<T>> ot {in_place};
   optional<optional<T>> ou {in_place, nullopt};
   optional<optional<T>> ov {optional<T>{}};
+
+  (void) ot;
+  (void) ou;
   
   optional<int> oi;
   auto ooi = make_optional(oi);