Fix the Ubuntu CI failures of the unity test build

- build unit-noexcept.cpp on its own: it suppresses GCC's -Wnoexcept
  around its include of the library, which has no effect once another
  file of the batch included the library first
- unit-comparison.cpp: make make_unordered_object inline
  (-Wmissing-prototypes in a named namespace) and replace the
  using-directive by using-declarations, as Clang reports a
  using-directive in an #include-d file (-Wheader-hygiene)
- unit-json_pointer-cpp20.cpp: include <compare> only with
  JSON_HAS_THREE_WAY_COMPARISON; GCC 9 and Clang 9 accept C++20 but
  have no <compare>

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-comparison.cpp

@@ -595,7 +595,7 @@ struct unordered_object_t : std::map<Key, Value, directed_less<Key>, Allocator>
 using unordered_json = nlohmann::json::with_object_t<unordered_object_t>;
 
 // the entries "0" to "9", enumerated in ascending or in descending order
-unordered_json make_unordered_object(const bool descending)
+inline unordered_json make_unordered_object(const bool descending)
 {
     unordered_json j = unordered_json::object_t(directed_less<std::string>(descending));
     for (int i = 0; i < 10; ++i)
@@ -676,7 +676,15 @@ using case_insensitive_map = std::map<Key, Value, case_insensitive_less, Allocat
 using ci_json = nlohmann::json::with_object_t<case_insensitive_map>;
 } // namespace unit_comparison_detail
 
-using namespace unit_comparison_detail; // NOLINT(google-build-using-namespace)
+// using-declarations rather than a using-directive, as this file may be
+// #include-d into a batch with other test files (JSON_TestUnityBuild)
+using unit_comparison_detail::ci_json;
+using unit_comparison_detail::innermost;
+using unit_comparison_detail::key_case_json;
+using unit_comparison_detail::key_case_less;
+using unit_comparison_detail::make_unordered_object;
+using unit_comparison_detail::nest;
+using unit_comparison_detail::unordered_json;
 
 TEST_CASE("equality of objects whose entries have no fixed order")
 {

tests/src/unit-json_pointer-cpp20.cpp

@@ -28,7 +28,9 @@ using nlohmann::json;
 #endif
 
 #ifdef JSON_HAS_CPP_20
-#include <compare>
+#if JSON_HAS_THREE_WAY_COMPARISON
+    #include <compare>
+#endif
 #include <string>
 #include <type_traits>