Make it possible for ECM to detect po files at configure time

Summary:
ECMPoQmTools will need to have the translations downloaded at configure time instead of on build time, otherwise it needs an awkward second configure.
To that end we introduce a KDE_L10N_SYNC_TRANSLATIONS variable that allows it to do so.
ECMPoTools will download the translations in the build directory, because cmake shouldn't touch the sources. Have it check the build directory too when the macro is called.

Test Plan: Built analitza translations.

Reviewers: Frameworks, cgiboudeaux

Reviewed By: cgiboudeaux

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

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D14569