[libc++] Fix linker script generation

Handle the case when libc++abi and libunwind are being built together
with libc++ in the runtimes build. This logic was used in the previous
implementation but dropped in r374116.

Differential Revision: https://reviews.llvm.org/D68791

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@374510 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/cmake/Modules/DefineLinkerScript.cmake b/cmake/Modules/DefineLinkerScript.cmake
index 213ab5d..2e68121 100644
--- a/cmake/Modules/DefineLinkerScript.cmake
+++ b/cmake/Modules/DefineLinkerScript.cmake
@@ -31,7 +31,9 @@
   set(link_libraries)
   if (interface_libs)
     foreach(lib IN LISTS interface_libs)
-      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 link_libraries "${CMAKE_LINK_LIBRARY_FLAG}$<TARGET_PROPERTY:${lib},OUTPUT_NAME>")
       else()
         list(APPEND link_libraries "${CMAKE_LINK_LIBRARY_FLAG}${lib}")