mk/tools: replace '!empty' conditions with '==' or '!='

Main work done by running 'pkglint -F --only "simpler" --only
"simplified" *.mk', with manual improvements afterwards:

* Removed several redundant ':U' modifiers, as pkglint doesn't know the
  exact order in which the infrastructure files include each other.

* Removed redundant 'defined(X11_TYPE)' and the corresponding comment.

(rillig)