reverse-ramdisk.c: set POSIX detection

c-programming/reverse-ramdisk.c

@@ -18,7 +18,14 @@ To-Do: error handling on line 184, function fread()
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <unistd.h>
+
+#if _POSIX_VERSION >= 200809L && defined(_POSIX_SYNCHRONIZED_IO)
+# include <fcntl.h>
+# include <unistd.h>
+# define IS_POSIX 1
+#else
+# define IS_POSIX 0
+#endif
 
 #ifdef DEBUG
 # define line_fail(x) do { \