cmake: Lets do not explicitely set a APPLICATION_SHORTNAME.
That makes it defaulting to the APPLICATION_NAME which is ownCloud.
That is what we want and expect to read the system exclude file from
on linux.
How we find the system exclude file linux:
ConfigFile::excludeFileFromSystem()
-> Theme::instance()->appName()
-> OwncloudTheme::appName() returns "ownCloud"
-> default Theme::appName() returns APPLICATION_SHORTNAME
-> cmake installs to ${SYSCONFDIR}/${APPLICATION_SHORTNAME}