re: https://github.com/Unidata/netcdf-c/issues/1502
If you plan to use any of the netcdf_XXX.h files, then it it is assumed
that you first include netcdf.h, followed by any netcdf_XXX.h files
you need. The EXTERNL (and other things) are defined in netcdf.h
so they should be available for use by the netcdf_XXX.h files.
This PR ensures that the above assumption is true. It also adds
some comments to that effect to the start of the netcdf_xxx.h files.
unlimited dimension hanging. Extending the size of an unlimited
dimension in HDF5 must be a collective operation, so now an error is
returned if trying to extend in independent access mode.
Quincey's bug fixes for parallel build portability, particularly
OpenMPI on MacOS-X.
