libarchive: Install COPYING with CMake documentation

Replay commit d39aee48 (Install COPYING with CMake documentation,
2011-06-16) after import of new libarchive snapshot.

Utilities/cmlibarchive/CMakeLists.txt

@@ -1081,3 +1081,5 @@ IF(APPLE)
 ENDIF(APPLE)
 
 add_subdirectory(libarchive)
+
+install(FILES COPYING DESTINATION ${CMake_DOC_DEST}/cmlibarchive)