Don't use C++20 path APIs unless we're building for C++20.

Fixes MSVC 2019 compatibility in C++17 mode.

Author: zrax

include/st_config.h.in

@@ -34,14 +34,17 @@
 #cmakedefine ST_HAVE_EXPERIMENTAL_STRING_VIEW
 #cmakedefine ST_HAVE_CXX17_FILESYSTEM
 #cmakedefine ST_HAVE_EXPERIMENTAL_FILESYSTEM
+#if (__cplusplus > 201703L) || (defined(_MSVC_LANG) && (_MSVC_LANG > 201703L))
 #cmakedefine ST_HAVE_CXX20_U8_FSPATH
 #endif
-#cmakedefine ST_ENABLE_STL_STRINGS
-#cmakedefine ST_ENABLE_STL_FILESYSTEM
+#endif
 #if (__cplusplus > 201703L) || (defined(_MSVC_LANG) && (_MSVC_LANG > 201703L))
 #cmakedefine ST_HAVE_CXX20_CHAR8_TYPES
 #endif
 
+#cmakedefine ST_ENABLE_STL_STRINGS
+#cmakedefine ST_ENABLE_STL_FILESYSTEM
+
 // Backwards compatibility for clients of string_theory
 #define ST_NOEXCEPT noexcept
 #define ST_OVERRIDE override