Remove the STATIC_LIBRARY option to fix static builds

Summary:
Previously static builds were broken because
GenerateExportHeader used:
get_property(type TARGET ${TARGET_LIBRARY} PROPERTY TYPE)
which returns STATIC_LIBRARY, however since we had an
option of the same name that variable was "dobule expanded"
in if statements, breaking GenerateExportHeader's logic

Additionally remove the legacy style elseif(...) and endif(...) form
conditionals

fixes https://bugs.kde.org/show_bug.cgi?id=418905

Reviewers: Kirigami, apol

Reviewed By: apol

Subscribers: apol, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D28066