message: Update linux README with CXXFLAGS
diff: @@ -262,7 +262,7 @@ Your Linux distribution might default to `gcc` instead of `clang`, if that's the
  $ mkdir out/cmake-release
  $ cd out/cmake-release
  # Or use a specific version of clang, for instance /usr/bin/clang-5.0
  -$ CC=/usr/bin/clang CXX=/usr/bin/clang++ \
  +$ CC=/usr/bin/clang CXX=/usr/bin/clang++ CXXFLAGS=-stdlib=libc++ \
  cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=../release/filament ../..
  ```
  