gt_wkt_srs.cpp: fix compilation with external libgeotiff. The file is dependant of quite a few CPL stuff, don't try to pretend otherwise

git-svn-id: https://svn.osgeo.org/gdal/trunk@26847 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx