add missing include and do not tests on clang-3.7

.github/workflows/ci.yml

@@ -32,12 +32,13 @@ jobs:
             os: ubuntu-18.04
             cxxflags: "cxxflags=--coverage -fsanitize=address,leak,undefined -fno-sanitize-recover=undefined"
             linkflags: "linkflags=--coverage -lasan -lubsan"
+            launcher: "testing.launcher=LD_PRELOAD=/usr/lib/x86_64-linux-gnu/libasan.so.5"
             gcov_tool: "gcov-10"
           - toolset: clang
-            compiler: clang++-3.7
+            compiler: clang++-3.8
             cxxstd: "03,11,14"
             os: ubuntu-16.04
-            install: clang-3.7
+            install: clang-3.8
           - toolset: clang
             compiler: clang++-9
             cxxstd: "03,11,14,17,2a"

examples/constexpr14_namespace_check.cpp

@@ -6,6 +6,8 @@
 
 #include <boost/config.hpp>
 
+#include <cstddef>
+
 template <class T>
 void do_something(const T&) {}