WMS: allow cache location to be specified with  GDAL_DEFAULT_WMS_CACHE_PATH configuration option if not provided in the XML (#4540, patch by vautieri)

git-svn-id: https://svn.osgeo.org/gdal/trunk@26900 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx