build-sys: Move to compiling with C11 support

This is needed for building with anonymous unions. A bunch of calls to
fail() that used to mysteriously work need fixing -- fail() is a macro
that takes a printf-style message as an argument. Not passing this
somehow worked with the previous compiler flags, but breaks with
-std=c11.

src/tests/cpu-mix-test.c

@@ -118,7 +118,7 @@ static void run_mix_test(
                     i,
                     samples[i], samples_ref[i],
                     samples0[i], samples1[i]);
-                fail();
+                ck_abort();
             }
         }
     }