Update c++ build environment.

Make use of the latest c++2a with the latest prebuilt clang and making
use of the libc++ from the same toolchain.

Change-Id: If0a9ff53b6fd5683236074ff689670da0a474608
diff --git a/src/api_test.cc b/src/api_test.cc
index fb6b8cb..cfc24a4 100644
--- a/src/api_test.cc
+++ b/src/api_test.cc
@@ -14,12 +14,16 @@
  * limitations under the License.
  */
 
+/*
+ * This order of headers works around a libc++ issue which prevents
+ * "atomic" being included before "stdatomic.h".
+ */
+#include <gmock/gmock.h>
+
 extern "C" {
 #include "hf/api.h"
 }
 
-#include <gmock/gmock.h>
-
 namespace
 {
 using ::testing::Eq;