Merge branch 'feature/clang-3.3' into develop

.travis.yml

@@ -1,4 +1,4 @@
-# Copyright 2016, 2017 Peter Dimov
+# Copyright 2016, 2017, 2018 Peter Dimov
 # Distributed under the Boost Software License, Version 1.0.
 # (See accompanying file LICENSE_1_0.txt or copy at http://boost.org/LICENSE_1_0.txt)
 
@@ -84,6 +84,22 @@ matrix:
           sources:
             - ubuntu-toolchain-r-test
 
+    - os: linux
+      compiler: /usr/bin/clang++
+      env: TOOLSET=clang COMPILER=/usr/bin/clang++ CXXSTD=11
+      addons:
+        apt:
+          packages:
+            - clang-3.3
+
+    - os: linux
+      compiler: /usr/bin/clang++
+      env: TOOLSET=clang COMPILER=/usr/bin/clang++ CXXSTD=11
+      addons:
+        apt:
+          packages:
+            - clang-3.4
+
     - os: linux
       compiler: clang++-3.5
       env: TOOLSET=clang COMPILER=clang++-3.5 CXXSTD=11,14,1z