automake: relax GNU standard directory style

GNU standard assumes `README` under root dir, while PR #403
didn't fix the building issue exactually.
https://www.gnu.org/software/automake/manual/html_node/List-of-Automake-options.html

Makefile.am

 ## Process this file with automake to produce Makefile.in
 
-AUTOMAKE_OPTIONS=subdir-objects
+AUTOMAKE_OPTIONS = subdir-objects foreign
 
 # Make sure that when we re-make ./configure, we get the macros we need
 ACLOCAL_AMFLAGS = -I m4