try building with libc++

.travis.yml

@@ -14,13 +14,17 @@ matrix:
           sources: &sources
             - llvm-toolchain-precise-3.8
             - ubuntu-toolchain-r-test
+            - libc++-dev
     - os: linux
-      env: COMPILER=clang++-3.8
+      env: COMPILER=clang++-3.8 CXXFLAGS=-stdlib=libc++
       addons:
         apt:
           packages:
             - clang-3.8
           sources: *sources
+    - os: osx
+      env: CXXFLAGS=-stdlib=libc++
 script: >
-    cmake -DCXXOPTS_BUILD_TESTS=ON -DCMAKE_CXX_COMPILER=$COMPILER .
+    cmake -DCXXOPTS_BUILD_TESTS=ON -DCMAKE_CXX_COMPILER=$COMPILER
+    -DCMAKE_CXX_FLAGS=$CXXFLAGS .
     && make && make ARGS=--output-on-failure test