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

[Nico Weber via cfe-commits]

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.

For compiler-rt's test runner, maybe it could check if the compiler does
find headers with `-isysroot $(xcrun -show-sdk-path)` added on OS X and
then just add that itself if it's needed and it helps.

On Fri, Sep 4, 2015 at 2:07 PM, Yuri Gribov via cfe-commits <
cfe-commits at lists.llvm.org> wrote:

> Oh, nice so this was actually done by Kuba to fix sanitizer unittests:
>
>       "echo '#include <iostream>' | ${COMPILER_RT_TEST_COMPILER} -E -x c++
> - > /dev/null"
>       "if [ $? != 0 ] "
>       "  then echo"
>       "  echo 'Your just-built clang cannot find C++ headers, which are
> needed to build and run compiler-rt tests.'"
>       "  echo 'You should copy or symlink your system C++ headers into
> ${LLVM_BINARY_DIR}/include/c++'"
>       "  if [ -d $(dirname $(dirname $(xcrun -f clang)))/include/c++ ]"
>       "    then echo 'e.g. with:'"
>       "    echo '  cp -r' $(dirname $(dirname $(xcrun -f
> clang)))/include/c++ '${LLVM_BINARY_DIR}/include/'"
>       "  elif [ -d $(dirname $(dirname $(xcrun -f clang)))/lib/c++ ]"
>       "    then echo 'e.g. with:'"
>       "    echo '  cp -r' $(dirname $(dirname $(xcrun -f clang)))/lib/c++
> '${LLVM_BINARY_DIR}/include/'"
>       "  fi"
>       "  echo 'This can also be fixed by checking out the libcxx project
> from llvm.org and installing the headers'"
>       "  echo 'into your build directory:'"
>       "  echo '  cd ${LLVM_SOURCE_DIR}/projects && svn co
> http://llvm.org/svn/llvm-project/libcxx/trunk libcxx'"
>       "  echo '  cd ${LLVM_BINARY_DIR} && make -C
> ${LLVM_SOURCE_DIR}/projects/libcxx installheaders
> HEADER_DIR=${LLVM_BINARY_DIR}/include'"
>       "  echo"
>       "  false"
>       "fi"
>
> What I mean is that we should either fix the tests, or describe precisely
>> the scenario in which libcxx+libcxxabi is needed.... because it's not
>> always needed. I suspect it's just needed when testing the sanitizers (but
>> I might be wrong).
>>
>
> What if I s/mandatory on OS X, optional otherwise/required to run tests on
> OS X/ ?
>
> -Y
>
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