Updates to qmake project file needed

[Stephen Lyons]

With the switch to Qt 5 as a minimum your Qt qmake file (qt5keychain.pri) will likely need updating as well, to include something like at least:

lessThan(QT_MAJOR_VERSION, 5) {
    error("qtkeychain requires Qt 5 or later")
}

[Frank Osterfeld]

Added the minimum version check, "qt5keychain.pri" is also a somewhat unfortunate name, probably it should just drop the "5" and be "qtkeychain.pri". That can break people's builds though...