Provide fallback for missing char8_t support

Clang 11.0.x with libc++ fails to compile tests in C++20 mode due to
incomplete char8_t support in std::filesystem::path.

Signed-off-by: Sergiu Deitsch <sergiu.deitsch@gmail.com>

Author: sergiud

include/nlohmann/detail/conversions/from_json.hpp

@@ -540,7 +540,7 @@ inline void from_json(const BasicJsonType& j, std_fs::path& p)
         JSON_THROW(type_error::create(302, concat("type must be string, but is ", j.type_name()), &j));
     }
     const auto& s = *j.template get_ptr<const typename BasicJsonType::string_t*>();
-#ifdef JSON_HAS_CPP_20
+#if defined(__cpp_lib_char8_t) && (__cpp_lib_char8_t >= 201907L)
     p = std_fs::path(std::u8string_view(reinterpret_cast<const char8_t*>(s.data()), s.size()));
 #else
     p = std_fs::u8path(s); // accepts UTF-8 encoded std::string in C++17, deprecated in C++20

include/nlohmann/detail/conversions/to_json.hpp

@@ -443,7 +443,7 @@ inline void to_json(BasicJsonType& j, const T& t)
 template<typename BasicJsonType>
 inline void to_json(BasicJsonType& j, const std_fs::path& p)
 {
-#ifdef JSON_HAS_CPP_20
+#if defined(__cpp_lib_char8_t) && (__cpp_lib_char8_t >= 201907L)
     const std::u8string s = p.u8string();
     j = std::string(s.begin(), s.end());
 #else

single_include/nlohmann/json.hpp

@@ -5325,7 +5325,7 @@ inline void from_json(const BasicJsonType& j, std_fs::path& p)
         JSON_THROW(type_error::create(302, concat("type must be string, but is ", j.type_name()), &j));
     }
     const auto& s = *j.template get_ptr<const typename BasicJsonType::string_t*>();
-#ifdef JSON_HAS_CPP_20
+#if defined(__cpp_lib_char8_t) && (__cpp_lib_char8_t >= 201907L)
     p = std_fs::path(std::u8string_view(reinterpret_cast<const char8_t*>(s.data()), s.size()));
 #else
     p = std_fs::u8path(s); // accepts UTF-8 encoded std::string in C++17, deprecated in C++20
@@ -6090,7 +6090,7 @@ inline void to_json(BasicJsonType& j, const T& t)
 template<typename BasicJsonType>
 inline void to_json(BasicJsonType& j, const std_fs::path& p)
 {
-#ifdef JSON_HAS_CPP_20
+#if defined(__cpp_lib_char8_t) && (__cpp_lib_char8_t >= 201907L)
     const std::u8string s = p.u8string();
     j = std::string(s.begin(), s.end());
 #else