PROJ linking to GDAL in Superbuild

PROJ is not linked to GDAL at link time.

In a SuperBuild environment, this means that the built proj library is not used (by default) and that a PROJ lib must exists somewhere in the system. For example this causes this bug on leod (macOS) : https://cdash.orfeo-toolbox.org/testDetails.php?test=1269313&build=1588

The problem can be solve by installing the proj library on the system, but maybe it would be nice to use the --with-static-proj4 gdal flag in the SuperBuild so that PROJ is linked at link time.

see https://lists.osgeo.org/pipermail/gdal-dev/2006-June/009114.html