Add ECMGenerateExportHeader, for improved handling of deprecated API

Summary:
Generates additional macros in the export header which can be used for
fine-grained disabling of warnings & visibility as well as excluding from
the build.

Reviewers: Frameworks, Build System

Subscribers: chehrlic, dfaure, cgiboudeaux, kde-frameworks-devel, kde-buildsystem

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D23789