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/mult-s16-test.c

@@ -65,7 +65,7 @@ START_TEST (mult_s16_test) {
 
         if (a != b) {
             pa_log_debug("%d: %d != %d", i, a, b);
-            fail();
+            ck_abort();
         }
     }