build: fix the build where install prefix is not user-writable

Summary:

• Validate only the files and symlinks
• Fix target names, they were swapped
• generate the icons in build-dir and then install it

Test Plan: done make and make install and verified install_manifest.txt looks sane

Reviewers: ngraham, lbeltrame, sitter

Reviewed By: sitter

Subscribers: sitter, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D27614