message: only use libc++ when building the project, and not while building libc++ itself
diff: @@ -48,24 +48,21 @@ matrix:
  include:
  - os: linux
  compiler: "clang-3.9"
  - env: CLANG_VERSION=3.9 BUILD='Debug' CC=clang-3.9 CXX=clang++-3.9
  - CXXFLAGS="-stdlib=libc++ ${CXXFLAGS}"
  + env: CLANG_VERSION=3.9 BUILD='Debug' CC=clang-3.9 CXX=clang++-3.9 LIBCXX=On
  addons: *clang39
  - os: linux
  compiler: "clang-4.0"
  - env: CLANG_VERSION=4.0 BUILD='Debug' CC=clang-4.0 CXX=clang++-4.0
  - CXXFLAGS="-stdlib=libc++ ${CXXFLAGS}"
  + env: CLANG_VERSION=4.0 BUILD='Debug' CC=clang-4.0 CXX=clang++-4.0 LIBCXX=On
  addons: *clang40
  - os: linux
  compiler: "clang-5.0"
  - env: CLANG_VERSION=5.0 BUILD='Debug' CC=clang-5.0 CXX=clang++-5.0
  + env: CLANG_VERSION=5.0 BUILD='Debug' CC=clang-5.0 CXX=clang++-5.0 LIBCXX=On
  CLANG_TIDY=clang-tidy-5.0
  - CXXFLAGS="-stdlib=libc++ ${CXXFLAGS}"
  addons: *clang50
  - os: linux
  compiler: "clang-5.0"
  - env: CLANG_VERSION=5.0 BUILD='Debug' CC=clang-5.0 CXX=clang++-5.0
  - CXXFLAGS="-stdlib=libc++ -fsanitize=address,undefined,integer -fno-sanitize-recover=all -fno-omit-frame-pointer ${CXXFLAGS}"
  + env: CLANG_VERSION=5.0 BUILD='Debug' CC=clang-5.0 CXX=clang++-5.0 LIBCXX=On
  + CXXFLAGS="-fsanitize=address,undefined,integer -fno-sanitize-recover=all -fno-omit-frame-pointer ${CXXFLAGS}"
  LDFLAGS="-fsanitize=address,undefined,integer"
  addons: *clang50
  - os: linux
  @@ -87,6 +84,10 @@ before_install:
  - if [ -n "$CLANG_VERSION" ]; then sudo PATH="${PATH}" CXX="$CXX" CC="$CC" ./ci_support/install_libcxx.sh; fi
  install:
  + if [ "$LIBCXX" == "On" ]; then
  + CXX_FLAGS="${CXX_FLAGS} -stdlib=libc++ -nostdinc++ -cxx-isystem /usr/include/c++/v1/ -Wno-unused-command-line-argument"
  + CXX_LINKER_FLAGS="${CXX_LINKER_FLAGS} -lc++abi"
  + fi
  if [ "$TRAVIS_OS_NAME" = "linux" ]; then
  sudo apt-get install -qq qt59base qt59declarative qt59quickcontrols2; source /opt/qt59/bin/qt59-env.sh;
  else
  