Use default gtags backend unless GTAGSLABEL is set

[Apteryks]

Hello,

I'd like to suggest leaving gtags using its default native backend instead of suggesting the user to use ctags via a prompt when the GTAGSLABEL environment variable is not set.

Rationale:

1. GTAGSLABEL=ctags gtags doesn't support reference tags, which can cause confusion (see: #151)
2. GTAGSLABEL=default gtags (or not specifying it at all) appears to be faster while generating reference tags.

Or was there a good reason why ctags shoud be preferred above Global's own default that I missed?

Thanks.