make ExternalLibraries/HDF5 compile with gcc-10

[Roland Haas]

The h5check utility that we include in ExternalLibrries/HDF5 (not part of the HDF5 library itself) fails to compile with gcc-10 unless -fcommon is passed, or it is patched as in https://github.com/EinsteinToolkit/ExternalLibraries-HDF5/pull/1

[Roland Haas]

Please review.

[Roland Haas]

Unless objected I will apply this after 2020-07-26

[Roland Haas]

Applied as git hash 462fce2 "HDF5: update included tarball to 1.12" of ExternalLibraries-HDF5

[Roland Haas]

Backported to ET_2020_05 as git hash 9f27e9d "HDF5: update included tarball to 1.12" of ExternalLibraries-HDF5