gnu global: set path of default gtags.conf in wrapper

[lpenz]

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.

Motivation for this change

The gtagslabel option was not working - it was being silently ignored.
That would be reported when the -v flag was used, though.

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[mention-bot]

@lpenz, thanks for your PR! By analyzing the history of the files in this pull request, we identified @codyopel, @pSub and @jwiegley to be potential reviewers.