Because it's a dependency of the plugin not of the application.
After so many attempts without luck, the first attempt I try after writing this reply is a dfferent error :)
So the boost is no longer my problem.
For other people that my face the same problem as me, in the includepath just add: '$(BOOST_ROOT)'
@JKSH, @hskoglund, thank you very much! Using both qt.conf file and "QMAKE_LFLAGS += -Wl,--rpath=\$$ORIGIN/lib" line in my app's .pro file helped me.
On it. Merci!
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