Drop automake portability warnings

We have an issue with recursive variable expansion for starpu.mk's use of
e.g. $(V_foo_$(AM_DEFAULT_VERBOSITY)), which clutters the output and hide
useful warnings

configure.ac

@@ -64,7 +64,7 @@ AC_SUBST([LIBSOCL_INTERFACE_AGE])
 
 AC_CANONICAL_SYSTEM
 
-AM_INIT_AUTOMAKE([1.11 -Wall foreign silent-rules color-tests parallel-tests subdir-objects])
+AM_INIT_AUTOMAKE([1.11 -Wall -Wno-portability foreign silent-rules color-tests parallel-tests subdir-objects])
 
 m4_ifdef([AM_SILENT_RULES],
   [AM_SILENT_RULES(yes)])