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/interpol-test.c

@@ -144,7 +144,7 @@ static void context_state_callback(pa_context *c, void *userdata) {
         case PA_CONTEXT_FAILED:
         default:
             pa_log_error("Context error: %s", pa_strerror(pa_context_errno(c)));
-            fail();
+            ck_abort();
     }
 }