Fix usage of query_qmake: differ between calls expecting qmake or not

Summary:
when KDE_INSTALL_USE_QT_SYS_PATHS has been explicitely set,
qmake can be considered a required dependency, otherwise the
paths will not be known, which would be unexpected.
Also does the code calling query_qmake, besides the one testing
for the same install prefix, not handle the case of empty strings
being returned and then results in bogus behaviour.

Thus this patch makes code fail hard if query_qmake is expected
to yield a result, but no qmake executable is found.

Reviewers: Frameworks, ltoscano, rdieter, apol

Reviewed By: apol

Subscribers: Build System

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D6772