As long as okular does not depend on kf5.42, please keep the compatibility with the old library as others did

Let's see if Aleix agrees with that or he has a different idea on how to fix this mess. Because the stable version of Okular has the same problem and we can't obviously change the dependencies there.

As mentioned in https://bugs.kde.org/show_bug.cgi?id=389224#c2 , there is the following new commit for Purpose that should fix the build when the "old" name is used for the dependency: https://cgit.kde.org/purpose.git/commit/?id=20a304236100b29014403a894cbda2d93ab28e41

I changed it so KDEExperimental still works with newer Purpose. I'd say let's keep the old one and when it's released again, then just change to just KF5::Purpose.

with current git-master purpose compilled and running I still fail to compile Okular:

CMake Warning at /home/tcanabrava/Projects/install/lib64/cmake/KDEExperimentalPurpose/KDEExperimentalPurposeConfig.cmake:2 (message):

Look up the Purpose framework using KF5Purpose

Call Stack (most recent call first):

CMakeLists.txt:71 (find_package)

CMake Error at /home/tcanabrava/Projects/install/lib64/cmake/KDEExperimentalPurpose/KDEExperimentalPurposeConfig.cmake:5 (string):

string no output variable specified

Call Stack (most recent call first):

CMakeLists.txt:71 (find_package)