GNU global's gtags and global tools were unable to find the default
gtags.conf, necessary to use gtagslabels. The error could be seen
when gtags was invoked with -v:
$ gtags -v --gtagslabel=pygments
[Sun Mar 19 14:13:17 UTC 2017] Gtags started.
Using default configuration.
GTAGSLABEL(--gtagslabel) ignored since configuration file not found.
Using 'gtags.files' as a file list.
[Sun Mar 19 14:13:17 UTC 2017] Creating 'GTAGS' and 'GRTAGS'.
[Sun Mar 19 14:13:17 UTC 2017] Done.
The wrapper now points to the default gtags.conf using the
GTAGSCONF environment variable.
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