Finally fixed it, i used a .qmake.conf file to store the root directory in a shared variable, for those who are interested, here are my 3 .pro files :
project.pro
TEMPLATE = subdirs
CONFIG += ordered c++11
SUBDIRS = ircbot \
myApp
myApp.depends = ircbot
mylib.pro
DESTDIR = $$lib_dir
TARGET = mylib
TEMPLATE = lib
CONFIG += staticlib
SOURCES += ...
HEADERS += ...
FORMS += ...
myApp.pro
DESTDIR = $$bin_dir
TARGET = myApp
TEMPLATE = app
LIBS += $$lib_dir/libmylib.a
INCLUDEPATH += ../mylib
SOURCES = ...
HEADERS += ...
FORMS += ...
and finally, the .qmake.conf file :
lib_dir=$$PWD/lib
bin_dir=$$PWD/bin
this successfully creates this project structure :
bin
myApp.exe
lib
mylib.a
project.pro
.qmake.conf
myApp
myApp.pro
mylib
mylib.pro
+ a build directory anywhere i want