[libcxxabi] Depend directly on unwind when not building standalone

[phosek]

Summary

When using libunwind and not building as standalone project, we can directly depend on the unwind library target.

[phosek]

The previous solution doesn't correctly handle transitive dependencies, while this one does.

[rmaprath]

I suppose this would fix the current build failures? e.g. http://lab.llvm.org:8011/builders/libcxx-libcxxabi-libunwind-x86_64-linux-debian/builds/135

In which case I think you can commit this. We are also seeing this failure on our downstream builders.

[phosek]

Yes, this should fix those failures.

libcxxabi/trunk/src/CMakeLists.txt

 if (LIBCXXABI_ENABLE_THREADS)
   append_if(libraries LIBCXXABI_HAS_PTHREAD_LIB pthread)
 endif()
 
 append_if(libraries LIBCXXABI_HAS_C_LIB c)
 
 if (LIBCXXABI_USE_LLVM_UNWINDER)
   if (NOT LIBCXXABI_BUILT_STANDALONE)
-    link_directories(${LLVM_LIBRARY_DIR})
+    if (LIBUNWIND_ENABLE_SHARED)
+      list(APPEND libraries unwind_shared)
+    else()
+      list(APPEND libraries unwind_static)
     endif()
+  else()
     list(APPEND libraries unwind)
+  endif()
 else()
   append_if(libraries LIBCXXABI_HAS_GCC_S_LIB gcc_s)
 endif()
 
 # Setup flags.
 append_if(LIBCXXABI_COMPILE_FLAGS LIBCXXABI_HAS_FPIC_FLAG -fPIC)
 append_if(LIBCXXABI_LINK_FLAGS LIBCXXABI_HAS_NODEFAULTLIBS_FLAG -nodefaultlibs)