cmake: warn user when QCA_SUFFIX is not set

Many linux distribution provides both version of Qt: Qt4 and Qt5.
These versions are binary is incompatible and any qt-based library
compiled against such Qt versions will be binary incompatible with
one another. In common case library name must not be related with
based libraries. In our case Linux users want to have both versions
QCA installed in their system. I will do not implicity rename Qt5
based QCA but just warn user and allow him explicity set QCA_SUFFIX.