HDF5 Fix, master branch (2020.11.24.)

Fixed the HDF5 pkg-config files for CentOS 7.

I now switched from removing the pkg-config files completely, to fixing them up instead. Since as it turns out, the files generated by HDF5 were already relocatable. It's just that CentOS 7's pkg-config version is too old to handle them correctly. 😦 So the build now "simplifies" HDF5's files to work correctly/reasonably on CentOS 7.

Note that SFT builds HDF5 very differently as part of the LCG release. (They don't use CMake for the build of it, but automake.) As a result their installation needs to be manually updated during installation to refer to the correct paths internally.

Thanks to @tadej for reminding me that we still had this issue in the master nightlies...