use __has_include(<filesystem>)) to detect if std::filesystem can be used

libi2pd/FS.h

@@ -17,12 +17,11 @@
 
 #ifndef STD_FILESYSTEM
 #	if (!defined(MAC_OSX) && !TARGET_OS_SIMULATOR && \
-	(__cplusplus >= 201703L) && defined(__cpp_lib_filesystem)) // C++ 17 or higher supporting std::filesystem
+	(__cplusplus >= 201703L) && __has_include(<filesystem>)) // C++ 17 or higher and supports std::filesystem
 #		define STD_FILESYSTEM 1
 #	else
 #		define STD_FILESYSTEM 0
 #	endif
-
 #endif
 
 namespace i2p {