cmake: fixed when set empty CMAKE_INSTALL_PREFIX in command line
cmake: fixed when set empty CMAKE_INSTALL_PREFIX in command line

JoshuaKolden/qca,karolherbst/qca,oktavarium/qca,Bjoe/qca,karolherbst/qca,oktavarium/qca,karolherbst/qca,JoshuaKolden/qca,JoshuaKolden/qca,KDE/qca,Bjoe/qca,Bjoe/qca,KDE/qca,karolherbst/qca,KDE/qca,Bjoe/qca,oktavarium/qca,JoshuaKolden/qca,oktavarium/qca,KDE/qca