I'm afraid that I can't see why it works on 12 and not on 11. Perhaps you could try -DCMAKE_SKIP_RPATH:BOOL=OFF. This might not be the solution, but it might help shed light on what is happenning.
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I have a repository with Qt5 for SLE 11 and 12. I have been added qmmp into that. One of the dependencies is soxr.
Repository: https://build.opensuse.org/package/show/home:linux4humans:qt5.6/sorx
Build log (SLE 11): https://build.opensuse.org/package/live_build_log/home:linux4humans:qt5.6/sorx/SLE_11_SP4/x86_64
Build log (SLE 12): https://build.opensuse.org/package/live_build_log/home:linux4humans:qt5.6/sorx/SLE_12_SP1/x86_64
I'm afraid that I can't see why it works on 12 and not on 11. Perhaps you could try -DCMAKE_SKIP_RPATH:BOOL=OFF. This might not be the solution, but it might help shed light on what is happenning.