Arch Linux

I believe this would be a problem with qloud, not a problem with qwt.

With 6.1.1 "-lqwt" is found.

Right, the libs moved to be consistent with the rest of the files. qmake should be picking up the new lib dir. I'm not that familiar with qmake, so I'm not sure what the mechanism here is.

qloud qmake (.pro) file contains standard libraries list:

LIBS += -lsndfile \
-lfftw3 \
-ljack \
-lqwt

You see, fftw, jack and qwt-6.1.1 libraries are found, 6.1.2 isn't. I don't understand all that linking mechanics, but would conclude qwt 6.1.2 is presented in the system other way rather other libs. The problem is the official repo doesn't contain packages with executable program and depended on qwt to run it and see the library is found.

Add -L/usr/lib/qwt to that list, it should build. It's assuming qwt is in the linker's search path instead of looking it up like it should (again, not sure now qmake does that, though).

The question, then, will be if it finds it at run time. If not, we may need something in /etc/ld.so.conf.d, or to move them back.

Yes, it is possible to add

LIBPATH += /usr/lib/qwt

into qmake project file. But it means, we are forced to take a rule to bother with _all_ deps in a common - had some external lib got own specific lib path or not. Of course, if such way is intended (even broking simple rules for developers), well, we are forced to accept it :) But I still guess there is an installation steps making linker aware about paths it must use to look at libraries. Of course, it may be wrong.

https://bugs.archlinux.org/task/38590 - about move to separate dir.
https://bugs.archlinux.org/task/44549 - this one - about link time.
https://bugs.archlinux.org/task/44590 - about runtime.

@student975 instead of meddling with the Qwt include/lib locations yourself, let the prf file handle it and instead use:

CONFIG += qwt

This loads qwt.prf and sets the necessary options.

@rubenvb Thanks for the suggestion. Yes, it can be built, but on start we still get:

$ bin/qloud
bin/qloud: error while loading shared libraries: libqwt.so.6: cannot open shared object file: No such file or directory

And, at any case, is .prf file using is the most wide way among Linux distributions at such sitruations?

Yes, this is actually the preferred way of adding libraries to Qt projects, but it needs a prf file written by the library author. The Qwt author(s) have done so, and if the distribution is worth anything, it should install the prf file in a qmake-findable location.

The fix for the runtime error is available here:
https://bugs.archlinux.org/task/44590

It consists of telling the system to look in /usr/lib/qwt for libraries at runtime.

You can for now create a file in /etc/ld.so.conf.d containing only

/usr/lib/qwt

and you should be fine. Other temporary alternatives are
1. rebuilding th package with linked PKGBUILD
2. symlinking libqwt to /usr/lib
3. adding /usr/lib/qwt to LD_LIBRARY_PATH.

Your choice. All should keep working when the fix lands in a new package version.