[CMake] Fix standalone builds: workaround the cxx target not getting imported yet (unlike clang target)

[sgraenitz]

Summary

Handle standalone builds separately and print a warning if we have no libcxx.

[sgraenitz]

Update documentation: libc++ should be checked out when building for macOS

lldb/trunk/CMakeLists.txt

if(LLDB_INCLUDE_TESTS)
   if(NOT LLDB_BUILT_STANDALONE)
     list(APPEND LLDB_TEST_DEPS yaml2obj)
   endif()
 
   if(TARGET liblldb)
     list(APPEND LLDB_TEST_DEPS liblldb)
   endif()
 
+  # Add dependencies if we test with the in-tree clang.
+  # This works with standalone builds as they import the clang target.
   if(TARGET clang)
     list(APPEND LLDB_TEST_DEPS clang)
     if(APPLE)
+      # If we build clang, we should build libcxx.
+      # FIXME: Standalone builds should import the cxx target as well.
+      if(LLDB_BUILT_STANDALONE)
+        # For now check that the include directory exists.
+        set(cxx_dir "${LLVM_DIR}/../../../include/c++")
+        if(NOT EXISTS ${cxx_dir})
+          message(WARNING "LLDB test suite requires libc++ in llvm/projects/libcxx or an existing build symlinked to ${cxx_dir}")
+        endif()
+      else()
         list(APPEND LLDB_TEST_DEPS cxx)
       endif()
     endif()
+  endif()
 
   if(TARGET dsymutil)
     list(APPEND LLDB_TEST_DEPS dsymutil)
   endif()
 
   add_subdirectory(test)
   add_subdirectory(unittests)
   add_subdirectory(lit)

lldb/trunk/www/build.html

<div id="content">
             <p>Building on Mac OS X with Xcode is as easy as downloading the code and building the Xcode project or workspace:</p>
             <ul>
               <li><a href="download.html">Download</a> the lldb sources.</li>
               <li>Follow the code signing instructions in <b>lldb/docs/code-signing.txt</b></li>
               <li>In Xcode 3.x: <b>lldb/lldb.xcodeproj</b>, select the <b>lldb-tool</b> target, and build.</li>
               <li>In Xcode 4.x: <b>lldb/lldb.xcworkspace</b>, select the <b>lldb-tool</b> scheme, and build.</li>
             </ul>
             <h2>Building LLDB with CMake</h2>
-            <p> First download the LLVM, Clang, and LLDB sources. Refer to <a href="source.html">this page</a> for precise instructions on this step.</p>
+            <p> First download the LLVM, Clang, libc++ and LLDB sources. Refer to <a href="source.html">this page</a> for precise instructions on this step.</p>
             <p> Refer to the code signing instructions in <b>lldb/docs/code-signing.txt</b> for info on codesigning debugserver during the build.</p>
             <p> Using CMake is documented on the <a href="http://llvm.org/docs/CMake.html">Building LLVM with CMake</a> page.
             Ninja is the recommended generator to use when building LLDB with CMake.</p>
             <code>
               &gt; cmake $PATH_TO_LLVM -G Ninja
               <br />&gt; ninja lldb
             </code>
             <p>