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

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.