README: "quick build": Use "make axtls" after all.

"make deplibs" also builds libffi, and that requires GNU autotools. As
we use host libffi by default, skip requiring users to build it for now.

README.md

@@ -71,7 +71,7 @@ Alternatively, fallback implementation based on setjmp/longjmp can be used.
 To build (see section below for required dependencies):
 
     $ cd unix
-    $ make deplibs
+    $ make axtls
     $ make
 
 Then to give it a try: