make HDF5 thread safe

[smoors]

one of our users encountered the following error when running CDO (tested with CDO/2.0.5-gompi-2021b and CDO/1.9.10-gompi-2021a)

cdo    seltimestep (Warning): Using a non-thread-safe NetCDF4/HDF5 library in a multi-threaded environment may lead to erroneous results!
cdo    seltimestep (Warning): Use a thread-safe NetCDF4/HDF5 library or the CDO option -L to avoid such errors.
/user/brussel/106/vsc10678/cut_region.sh: line 42: 42420 Segmentation fault      cdo -seltimestep,1 -sellonlatbox,$WEST,$EAST,$SOUTH,$NORD $file ${file%.*}_${FILE_EXT}.nc

the reason is that HDF5, on which NetCDF4 depends, is not configured with --enable-threadsafe.

my suggestion would be to add options --enable-threadsafe --enable-unsupported, which would make the C API thread-safe. the option --enable-unsupported is needed to allow --enable-threadsafe to be used together with --enable-cxx, as --enable-threadsafe has no effect on the C++ APIs.

this seems a safe change, as argued in the similar conda-forge PR conda-forge/hdf5-feedstock#57 (comment)

• threadsafety only applies to the C API
• C++ / FORTRAN APIs are unaffected