cmSystemTools: Restore unreachable return to silence warnings

Some compilers do not recognize that the end of copy_data cannot
be reached and complain that there is no return statement.  Clang
warns that there is an unreachable return statement if it appears.
Conditionally add the return statement when not building with Clang.
Merge topic 'clang-warnings' into next

1de08685 cmSystemTools: Restore unreachable return to silence warnings