expand bsd license in cmake modules

Summary:
when revisiting the cmake module licensing during the early days of kf5
we realized that referencing this external file is less than ideal. for one
the license is super short anyway, so putting it in the header shouldn't
be a problem. additionally this external reference makes it hard to
copy finders into other source trees as they could (accidentally) forget
to copy the referenced file as well (unless they read the header).
thirdly the 'BSD' license is a bit ambiguous as there are various forms
of this license in circulation and they all get referred to as BSD license
despite having different conditions. (2-clause, 3-clause, 4-clause)

Reviewers: apol

Reviewed By: apol

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D20785