This enables support for e.g. setting CFLAGS, custom defines, etc.

Task-number: QTCREATORBUG-9380
Change-Id: I9d4a560ac5ba8a40c51c15fdf8009c0dba5eeef1
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>