make env variable overwrite safe_mode

git-svn-id: https://svn.mcs.anl.gov/repos/parallel-netcdf/trunk@1683 c368ceb8-fd28-0410-a457-e46ee1d7cd4a
revise erro code checking

git-svn-id: https://svn.mcs.anl.gov/repos/parallel-netcdf/trunk@1684 c368ceb8-fd28-0410-a457-e46ee1d7cd4a