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/CMakeLists.txt

@@ -55,6 +55,10 @@ elseif(CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
     )
 endif()
 
+if(UNITS_UDLS)
+    target_compile_definitions(mp-units-core INTERFACE UNITS_UDLS=1)
+endif()
+
 if(DEFINED UNITS_DOWNCAST_MODE)
     set(downcast_mode_options OFF ON AUTO)
     list(FIND downcast_mode_options "${UNITS_DOWNCAST_MODE}" downcast_mode)

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>{};