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-volume-test.c

@@ -78,7 +78,7 @@ static void run_volume_test(
                 pa_log_debug("Correctness test failed: align=%d, channels=%d", align, channels);
                 pa_log_debug("%d: %04hx != %04hx (%04hx * %08x)\n", i, samples[i], samples_ref[i],
                         samples_orig[i], volumes[i % channels]);
-                fail();
+                ck_abort();
             }
         }
     }