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!