Avoid potential deadlock

git-svn-id: https://svn.osgeo.org/gdal/trunk@25577 f0d54148-0727-0410-94bb-9a71ac55c965
Make HDF4 driver thread-safe by adding a global mutex (since the HDF4 library isn't thread-safe)

git-svn-id: https://svn.osgeo.org/gdal/trunk@25578 f0d54148-0727-0410-94bb-9a71ac55c965