build: Added an option to disable UDLs support

Now it is time to get rid of UDLs from tests and examples and then disable them by default.

src/core/include/units/generic/angle.h

@@ -43,6 +43,8 @@ concept Angle = QuantityOfT<T, dim_angle>;
 template<UnitOf<dim_angle<>> U, Representation Rep = double>
 using angle = quantity<dim_angle<>, U, Rep>;
 
+#if UNITS_UDLS
+
 inline namespace literals {
 
 // rad
@@ -51,6 +53,8 @@ constexpr auto operator"" _q_rad(long double l) { return angle<radian, long doub
 
 }  // namespace literals
 
+#endif // UNITS_UDLS
+
 namespace references {
 
 inline constexpr auto rad = reference<dim_angle<>, radian>{};