[ECMGenerateHeaders] Add option for other header file extension than .h

Summary:
Quite some C++-based projects do not use .h as header file extension, but
.hpp, .hxx or other variants. Making the header file extension configurable
enables to make use of ECMGenerateHeaders in such projects.

Test Plan:
The added unit test works, existing unit tests work as before.
Also using in project with .hpp files works.

Reviewers: apol

Reviewed By: apol

Subscribers: apol, kde-frameworks-devel, kde-buildsystem

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D13721