title: clang++ installed via homebrew (macOS): compilation errors
body: <p>After installing clang++ (tried v. 6.0.1 and 7.0) with:<br>
  <code>brew install --with-toolchain llvm</code></p>
  
  <p>very trivial programs result to the following error:</p>
  
  <pre><code> In file included from test.cpp:1:
   In file included from /usr/local/Cellar/llvm/7.0.0/include/c++/v1/iostream:38:
   In file included from /usr/local/Cellar/llvm/7.0.0/include/c++/v1/ios:215:
   In file included from /usr/local/Cellar/llvm/7.0.0/include/c++/v1/iosfwd:90:
   /usr/local/Cellar/llvm/7.0.0/include/c++/v1/wchar.h:119:15: fatal error: 'wchar.h' file not found
   #include_next &lt;wchar.h&gt;
  </code></pre>
  
  <p>Command used to compile:</p>
  
  <pre><code> clang++7() {
      LDFLAGS="-L/usr/local/opt/llvm/lib -Wl,-rpath,/usr/local/opt/llvm/lib"
      CPPFLAGS="-I/usr/local/opt/llvm/include"
      /usr/local/opt/llvm/bin/clang++ -std=c++11 $CPPFLAGS $LDFLAGS $1
   }
  </code></pre>
  
  <p>Is it possible to use the official clang instead of Apple's version?<br>
  With Apple's version, we do not even know which version of LLVM it really is...</p>