[CMake][libcxx] Do not rely on the existence of c++abi or unwind targets

[phosek]

Summary

There is no guaranteed order in which CMake files for individual runtimes are invoked and therefore we cannot rely on existence of targets defined in other runtimes. Use the new HAVE_<name> options instead in those cases.

[beanz]

LGTM

libcxx/trunk/CMakeLists.txt

 if (LIBCXX_CXX_ABI STREQUAL "default")
   find_path(
     LIBCXX_LIBCXXABI_INCLUDES_INTERNAL
     cxxabi.h
     PATHS ${LLVM_MAIN_SRC_DIR}/projects/libcxxabi/include
           ${LLVM_MAIN_SRC_DIR}/runtimes/libcxxabi/include
     NO_DEFAULT_PATH
   )
-  if (NOT DEFINED LIBCXX_STANDALONE_BUILD AND
-      IS_DIRECTORY "${LIBCXX_LIBCXXABI_INCLUDES_INTERNAL}")
+  if (IS_DIRECTORY "${LIBCXX_LIBCXXABI_INCLUDES_INTERNAL}")
     set(LIBCXX_CXX_ABI_LIBNAME "libcxxabi")
     set(LIBCXX_CXX_ABI_INCLUDE_PATHS "${LIBCXX_LIBCXXABI_INCLUDES_INTERNAL}")
     set(LIBCXX_CXX_ABI_INTREE 1)
   else()
     if (WIN32 AND NOT MINGW)
       # FIXME: Figure out how to configure the ABI library on Windows.
       set(LIBCXX_CXX_ABI_LIBNAME "none")
     else()

libcxx/trunk/lib/CMakeLists.txt

if (LIBCXX_ENABLE_STATIC)
   )
   list(APPEND LIBCXX_TARGETS "cxx_static")
   # Attempt to merge the libc++.a archive and the ABI library archive into one.
   if (LIBCXX_ENABLE_STATIC_ABI_LIBRARY)
     set(MERGE_ARCHIVES_SEARCH_PATHS "")
     if (LIBCXX_CXX_ABI_LIBRARY_PATH)
       set(MERGE_ARCHIVES_SEARCH_PATHS "-L${LIBCXX_CXX_ABI_LIBRARY_PATH}")
     endif()
-    if (TARGET ${LIBCXX_CXX_ABI_LIBRARY})
+    if ((TARGET ${LIBCXX_CXX_ABI_LIBRARY}) OR
+        (${LIBCXX_CXX_ABI_LIBRARY} STREQUAL "cxxabi(_static|_shared)?" AND HAVE_LIBCXXABI))
       set(MERGE_ARCHIVES_ABI_TARGET "$<TARGET_LINKER_FILE:${LIBCXX_CXX_ABI_LIBRARY}>")
     else()
       set(MERGE_ARCHIVES_ABI_TARGET
           "${CMAKE_STATIC_LIBRARY_PREFIX}${LIBCXX_CXX_ABI_LIBRARY}${CMAKE_STATIC_LIBRARY_SUFFIX}")
     endif()
     add_custom_command(TARGET cxx_static POST_BUILD
     COMMAND
       ${PYTHON_EXECUTABLE} ${LIBCXX_SOURCE_DIR}/utils/merge_archives.py

 # Generate a linker script inplace of a libc++.so symlink. Rerun this command
 # after cxx builds.
 if (LIBCXX_ENABLE_SHARED AND LIBCXX_ENABLE_ABI_LINKER_SCRIPT)
   # Get the name of the ABI library and handle the case where CXXABI_LIBNAME
   # is a target name and not a library. Ex cxxabi_shared.
   set(LIBCXX_INTERFACE_LIBRARY_NAMES)
   foreach(lib ${LIBCXX_INTERFACE_LIBRARIES})
     # FIXME: Handle cxxabi_static and unwind_static.
-    if (TARGET ${lib})
+    if (TARGET ${lib} OR
+        (${lib} MATCHES "cxxabi(_static|_shared)?" AND HAVE_LIBCXXABI) OR
+        (${lib} MATCHES "unwind(_static|_shared)?" AND HAVE_LIBUNWIND))
       list(APPEND LIBCXX_INTERFACE_LIBRARY_NAMES "$<TARGET_PROPERTY:${lib},OUTPUT_NAME>")
     else()
       list(APPEND LIBCXX_INTERFACE_LIBRARY_NAMES "${lib}")
     endif()
   endforeach()
   #split_list(LIBCXX_INTERFACE_LIBRARY_NAMES)
   # Generate a linker script inplace of a libc++.so symlink. Rerun this command
   # after cxx builds.