Qt5 cmake stuff

[Stephen Kelly]

Alexander Neundorf wrote:

> On Monday 14 May 2012, David Faure wrote:
>> On Monday 14 May 2012 23:13:44 Alexander Neundorf wrote:
>> > if (NOT TARGET Qt5::Core)
>> > 
>> >   add_library(Qt5::Core SHARED IMPORTED)
>> > 
>> > endif()
>> 
>> Thanks, that works, and is MUCH simpler too ;-)
>> 
>> Any reason against committing that upstream?
> 
> There was some subtle reason why this can have issues in some cases...
> Stephen, what was it exactly ?
> It was something with unexpected name clashes IIRC.
> We need to be able to notice if somebody accidentially created a target
> with the same name already, and instead of silently not creating the
> imported target, fail. Was that it ?
> If this way the reason, I'm not sure how realistic this is when we use
> "namespaced" imported targets as is done here (the "Qt5::").

I agree:

https://codereview.qt-project.org/#change,26253

Sorry about the breakage. I was following the tradition of breaking 
everything and then going on vacation. :)

Thanks,

Steve.