Use built-in CMake feature for requiring a specific language standard

This feature was added in CMake 3.8. Requiring specific language
features is an outdated approach that CMake stopped supporting in
favor of simply specifyin the specific language standard you want.

https://cmake.org/cmake/help/v3.10/prop_gbl/CMAKE_CXX_KNOWN_FEATURES.html

src/CMakeLists.txt

@@ -362,29 +362,10 @@ set_target_properties(Catch2 PROPERTIES
                                 VERSION ${PROJECT_VERSION}
                               SOVERSION ${PROJECT_VERSION})
 
-# depend on bunch of C++11 and C++14 features to have C++14 enabled by default
+# require C++14
 target_compile_features(Catch2
   PUBLIC
-    cxx_alignas
+    cxx_std_14
-    cxx_alignof
-    cxx_attributes
-    cxx_auto_type
-    cxx_constexpr
-    cxx_defaulted_functions
-    cxx_deleted_functions
-    cxx_final
-    cxx_lambdas
-    cxx_noexcept
-    cxx_override
-    cxx_range_for
-    cxx_rvalue_references
-    cxx_static_assert
-    cxx_strong_enums
-    cxx_trailing_return_types
-    cxx_unicode_literals
-    cxx_user_literals
-    cxx_variable_templates
-    cxx_variadic_macros
 )
 
 configure_file(
@@ -475,26 +456,7 @@ if (CATCH_BUILD_EXAMPLES OR CATCH_BUILD_EXTRA_TESTS)
     )
     target_compile_features(Catch2_buildall_interface
       INTERFACE
-        cxx_alignas
+        cxx_std_14
-        cxx_alignof
-        cxx_attributes
-        cxx_auto_type
-        cxx_constexpr
-        cxx_defaulted_functions
-        cxx_deleted_functions
-        cxx_final
-        cxx_lambdas
-        cxx_noexcept
-        cxx_override
-        cxx_range_for
-        cxx_rvalue_references
-        cxx_static_assert
-        cxx_strong_enums
-        cxx_trailing_return_types
-        cxx_unicode_literals
-        cxx_user_literals
-        cxx_variable_templates
-        cxx_variadic_macros
     )
 endif()