Feature 'tslib' was enabled, but the pre-condition 'libs.tslib' failed (Qt5 cross-compile)

Question

I cross-compiled Qt5.12.11 for arm without the tslib support, and it was successful. Now, I need to add the tslib support. I have the tslib already cross-compiled for my target architecture. I add the -tslib to my configure in this way, specifying the include and lib path with -I and -L:

./configure -opensource -confirm-license -xplatform arm-cortex_a8-linux-gnueabi-g++ -prefix myInstallDir/ -no-opengl -no-pch -I /path/to/tslib/include/ -L /path/to/tslib/lib/ -tslib

In the configure.log I can see the following:

loaded result for library config.qtbase_gui.libraries.tslib
Trying source 0 (type inline) of library tslib ...
None of [libts.so libts.a] found in [] and global paths.
  => source produced no result.
test config.qtbase_gui.libraries.tslib FAILED

I tryied to add in my qtbase/mkspecs/arm-cortex_a8-linux-gnueabi-g++/qmake.conf the following lines:

QMAKE_INCDIR +=  /path/to/tslib/include/
QMAKE_LIBDIR +=  /path/to/tslib/lib/

without any success. I also tryied to add /path/to/tslib/lib to LD_LIBRARY_PATH environment path, but nothing changes. is it a problem related to the localization of the tslib lib & include paths? can anyone give me a suggestion please? Thank you.