GNU standard targets

[jdahm]

One thing I have missed with CMake is the ability to rely on GNU standard targets. 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...