Update .travis.yml and setup_variables.sh

.travis.yml

@@ -30,6 +30,8 @@ addons:
     - php
     - cmake
     - g++
+    - clang
+    - libc++-dev
     - ccache
     - maven
     - python3

travis/setup_variables.sh

@@ -39,6 +39,9 @@ elif [[ "$TRAVIS_OS_NAME" == "linux" ]]; then
   export JAVA_HOME="/usr/lib/jvm/java-11-openjdk-$TRAVIS_CPU_ARCH_JAVA"
   export JAVA_INCLUDE_PATH="/usr/lib/jvm/java-11-openjdk-$TRAVIS_CPU_ARCH_JAVA/include"
   export CPU_CORES=" -- -j${CPU_CORES_NUM}"
+  export CXXFLAGS="-stdlib=libc++"
+  export CC="/usr/bin/clang"
+  export CXX="/usr/bin/clang++"
 fi
 
 # ====== Print variables