fix: some fixes revealed during clang compilation

example/total_energy.cpp

@@ -48,7 +48,7 @@ void si_example()
   using namespace mp_units::si::unit_symbols;
   constexpr auto GeV = si::giga<si::electronvolt>;
   constexpr QuantityOf<isq::speed> auto c = 1. * si::si2019::speed_of_light_in_vacuum;
-  constexpr auto c2 = pow<2>(c);
+  auto c2 = pow<2>(c);
 
   const auto p1 = isq::momentum(4. * GeV / c);
   const QuantityOf<isq::mass> auto m1 = 3. * GeV / c2;