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

This relands commit r291727.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@292085 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CMakeLists.txt b/lib/CMakeLists.txt
index 23a17dd..e11ff26 100644
--- a/lib/CMakeLists.txt
+++ b/lib/CMakeLists.txt
@@ -247,7 +247,8 @@
     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
@@ -318,7 +319,9 @@
   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}")