[PATCH][www] Mention that libcxx is necessary on OS X

[Kuba Brecka via cfe-commits]

> On 07 Sep 2015, at 17:21, Nico Weber <thakis at chromium.org> wrote:
> 
> libcxx is needed to run compiler-rt tests, but it shouldn't be needed to run clang's tests. And to _use_ your self-built compiler, you need to add `-isysroot $(xcrun -show-sdk-path)` to the compile flags to let the compiler find C headers anyway, which also makes it find the libc++ headers.

No, it doesn’t:

   $ clang++ a.cpp  # System version.
   $ clang++ a.cpp -isysroot $(xcrun --show-sdk-path)  # System version.
   $ ./clang++ ~/test/cpp101.cpp  # Self-built version.
       a.cpp:1:10: fatal error: 'iostream' file not found
   $ ./clang++ a.cpp -isysroot $(xcrun --show-sdk-path)  # Self-built version.
       a.cpp:1:10: fatal error: 'iostream' file not found

Kuba