Lintian tag information: debian-rules-uses-special-shell-variable (type: warning)

Description (from lintian-explain-tags)

The rules file use the make variable $(_).

According to Policy 4.9, 'invoking either of make -f debian/rules <...> or
./debian/rules <args...>' must result in identical behavior'. The $_
variable is an easy way to violate that rule unwittingly.

If the rules file uses $(dir $(_)) to discover the directory containing
the source package (for example, in order to implement the get-orig-source
target) please replace it with $(dir $(firstword $(MAKEFILE_LIST))).

Please refer to Main building script: debian/rules (Section 4.9) in the
Debian Policy Manual and https://stackoverflow.com/a/27628164 for details.

Visibility: warning
Show-Always: no
Check: debian/rules
Renamed from: debian-rules-should-not-use-underscore-variable

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