When CMake runs, configure_file() will clobber the config files in
the build/ directory, which is annoying during testing: you need
to keep making the same edits, or edit the source.
- Introduce new behavior: the config file is **not** overwritten unless
the source file is newer. This means that edits to config files
in the build directory are preserved.
- If INSTALL_CONFIG is **on** then the files are clobbered anyway (the
source is considered new regardless).