Fix linker script generation for in-tree builds

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@283700 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/Modules/HandleLibcxxFlags.cmake

@@ -188,6 +188,14 @@ macro(add_library_flags_if condition)
   endif()
 endmacro()
 
+# Add a list of libraries or link flags to 'LIBCXX_LIBRARIES'.
+macro(add_interface_library)
+  foreach(lib ${ARGN})
+    list(APPEND LIBCXX_LIBRARIES ${lib})
+    list(APPEND LIBCXX_INTERFACE_LIBRARIES ${lib})
+  endforeach()
+endmacro()
+
 # Turn a comma separated CMake list into a space separated string.
 macro(split_list listname)
   string(REPLACE ";" " " ${listname} "${${listname}}")