GNU standard targets

One thing I have missed with CMake is the ability to rely on [GNU standard targets](https://www.gnu.org/prep/standards/html_node/Standard-Targets.html). Could we try to add the ones which make sense back in with BLT? I would like to start with

- `make tags`: I like to import tags tables in my editor and while I can manually add a target or add a shell script to generate these, since BLT and CMake already know about all the source files, could we add this to BLT? I know some modern editors have their own implementation, but I still think it would be nice to have for those of us using vim/emacs. Cases for ctags and etags would suffice.

- `make uninstall`

@kennyweiss had a good point in #189 that some of these may not make sense for generators other than `make`, so we would have to look into how to exclude them for those generators...

Start by reviewing BLT's CMake target-generation logic and the GNU standard targets linked in the issue. Determine how to provide ctags, etags, and uninstall targets for Make generators without affecting other generators. Done means the applicable targets are available and their behavior is verified for the supported generator cases.

Written by the indexing model from the issue text.