Debug Xcode build failure

.travis.yml

@@ -21,6 +21,8 @@ before_install:
        brew link qt5 --force && \
        brew install protobuf && \
        ls -lR /usr/local/include/google/protobuf; \
+       which clang++ \
+       clang++ -E -x c++ - -v < /dev/null \
      fi"
 
 addons: