Basic cross-compiling support for parsetrigrams

Summary:
Works with or without KF5_HOST_TOOLING set. Not particularly elegant,
but before spending more time on this I think it is probably worth
investigating if we cannot avoid this step entirely, considering how
simple this tool is.

Reviewers: cullmann, apol

Reviewed By: cullmann, apol

Subscribers: apol, cullmann, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25143