ports-mgmt/portlint: Remove errors for use of :L and :U variable modifiers

[jrm]

Summary

ports-mgmt/portlint: Remove errors for use of :L and :U variable modifiers

In older versions of make included in the base, :L/:U were (deprecated)
variable modifiers to convert variables to lower/upper case. Both of these
modifiers now have different meanings in FreeBSD 10.3 and 11.0 make, so
remove the errors reported when they are used.

[mat]

whilst portlint's source is in the ports tree, the ports tree is not where it is maintained. You should probably open a PR against portlint with that change.

[jrm]

PR created. https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=218325

[jrm]

Committed in r439128.