+# ifndef FILE_SYSTEM_DRIVE_PREFIX_CAN_BE_RELATIVE
+# define FILE_SYSTEM_DRIVE_PREFIX_CAN_BE_RELATIVE 0
+# endif
+
+# ifndef DOUBLE_SLASH_IS_DISTINCT_ROOT
+# define DOUBLE_SLASH_IS_DISTINCT_ROOT 0
+# endif
+
+# if FILE_SYSTEM_DRIVE_PREFIX_CAN_BE_RELATIVE
+# define IS_ABSOLUTE_FILE_NAME(F) ISSLASH ((F)[FILE_SYSTEM_PREFIX_LEN (F)])
+# else
+# define IS_ABSOLUTE_FILE_NAME(F) \
+ (ISSLASH ((F)[0]) || 0 < FILE_SYSTEM_PREFIX_LEN (F))
+# endif