ProcessParameter: Ensure macro expansion is done in the main thread

The macro expanders are not thread safe. Also the values should be
expanded in init() as later changes to e.g. the buildconfiguration
should not affect the build anymore.

Change-Id: I82f5cd229d82cdb9f897c1db69c47b028cca29d1
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>