Installed geos-config does not use OTB_INSTALL_DIR

I faced errors while running pip install shapely --no-binary and I found out this is due to geos-config not returning the installation path for OTB (but prefix=/builds/otb/xdk).
It should use OTB_INSTALL_DIR, as in gdal-config. How is it generated ?

Or since the prefix is actually hard-coded in geos-config script, could it be a simple sed command in the post install script ?