Fix build with clang on libstdc++ systems.
https://lists.gnu.org/archive/html/bug-commoncpp/2019-05/msg00000.html
--- inc/ucommon/platform.h.orig	2015-11-07 12:15:59.000000000 -0600
+++ inc/ucommon/platform.h	2019-05-23 23:06:33.000000000 -0500
@@ -174,7 +174,7 @@
 #if __cplusplus <= 199711L && !defined(_MSC_VER)
 #if defined(__GNUC_MINOR__) && !defined(__clang__)
 #define nullptr __null
-#elif !defined(__clang__) || (defined(__clang__) && defined(__linux__))
+#elif !defined(__clang__) || (defined(__clang__) && !defined(_LIBCPP_VERSION))
 const class nullptr_t 
 {
 public: