refactor: got rid of gcc-9 backlog

BREAKING CHANGE: gcc-9.3 no longer supported

example/alternative_namespaces/units_str.h

@@ -4,7 +4,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 // get at the units text of the quantity, without its numeric value
-inline auto constexpr units_str(const units::Quantity AUTO& q)
+inline auto constexpr units_str(const units::Quantity auto& q)
 {
   typedef std::remove_cvref_t<decltype(q)> qtype;
   return units::detail::unit_text<typename qtype::dimension, typename qtype::unit>();