message: improve readme for building on linux
diff: @@ -230,6 +230,7 @@ If you use CMake directly instead of the build script, pass `-DENABLE_JAVA=OFF`
  Make sure you've installed the following dependencies:
  +- `clang-7`
  - `libglu1-mesa-dev`
  - `libc++-7-dev` (`libcxx-devel` on Fedora)
  - `libc++abi-7-dev`
  @@ -262,7 +263,7 @@ 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-7
  -$ CC=/usr/bin/clang CXX=/usr/bin/clang++ CXXFLAGS=-stdlib=libc++ LDFLAGS=-lc++abi \
  +$ CC=/usr/bin/clang CXX=/usr/bin/clang++ CXXFLAGS=-stdlib=libc++ \
  cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=../release/filament ../..
  ```
  @@ -271,10 +272,10 @@ solution is to use `update-alternatives` to both change the default compiler, an
  specific version of clang:
  ```
  -$ update-alternatives --install /usr/bin/cc cc /usr/bin/clang 100
  -$ update-alternatives --install /usr/bin/c++ c++ /usr/bin/clang++ 100
  $ update-alternatives --install /usr/bin/clang clang /usr/bin/clang-7 100
  $ update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-7 100
  +$ update-alternatives --install /usr/bin/cc cc /usr/bin/clang 100
  +$ update-alternatives --install /usr/bin/c++ c++ /usr/bin/clang++ 100
  ```
  Finally, invoke `ninja`:
  