Log Message:
Fixed SUBST sed expression that confused pkglint.

In the text "s|\$diffpath = .*|...|", the dollar is interpreted as a Make
variable, as if it were "s|\${d}iffpath". Since that variable is usually
not defined, the substitution failed to match. Since most systems provide
a diff tool in /usr/bin, this didn't break anything.

The patched version can also handle DIFF=/usr/bin/gdiff or
DIFF=/opt/bin/gnudiff.