Under Linux/MacOS

You just have to add the following lines in the .pro file:
CONFIG += link_pkgconfig
PKGCONFIG += ibex
as in the following screenshot:

Under Windows

You need to set up paths manually inside the .pro file.

We need two paths, the [ibex path] and [soplex path]

Typically, ibex path is :
C:\MinGW\msys\1.0\home\[user]\Ibex\ibex-2.0.6

and soplex path is :
C:\MinGW\msys\1.0\home\[user]\Ibex\soplex-1.7.0

If Ibex is install with Gaol (by default with 32 bits):

Insert in the bottom of the .pro file:

CXXFLAGS += -Wno-unused-parameter -Wno-unused-variable
INCLUDEPATH += [ibex path]\include\ibex [soplex path]\src
LIBS += -L[ibex path]\bin-L[soplex path]\lib -libex -lsoplex -lz -lultim

If Ibex is install with Filib (by default with 64 bits):

Insert in the bottom of the .pro file:

CXXFLAGS +=-frounding-math -msse2 -mfpmath=sse -Wno-unused-parameter -Wno-unused-variable
INCLUDEPATH += [ibex path]\include\ibex [soplex path]\src
LIBS += -L[ibex path]\bin-L[soplex path]\lib -libex -lsoplex -lz -lprim
Example with Windows+Goal:

If Ibex is installed in a local folder (Linux/MacOS)

Click in "Projects" on the left bar. A window "Build settings" should appear. Click on "Details" button on the bottom:

Then click on "Add" and enter the following variable:

Variable: PKG_CONFIG_PATH
Value: [prefix]/share/pkgconfig

where [prefix] is the path you specified with --prefix when configuring ibex.

It's done.