Must be enabled at compile time. It bends around the respective
naive implementations in OST to their parasail counterpart.
On one hand its faster and on the other hand it should fix potential
suboptimal alignments which are reported in SCHWED-6266

This commit additional drops more side-effects of the presence of a
seqres record by moving all the logic to the GetSeqRes function, rather
than being generated on the fly during parsing.

This replaces the awkward FindCompound(... by=) which would return
a single arbitrary compound.

This allows preferentially picking a non-obsolete compound
in FindCompound by="smiles/inchi"

Support for DSSP 4.0 (https://github.com/PDB-REDO/dssp) has been
enabled in commit da57a1f6.
This worked if CLIBD_MON env var points to some CCP4 related
compound library. In the example of dssp shipped with Debian
bullseye, that's not the case. The observation there:
It works if applied on a PDB file downloaded from RCSB with all bells
and whistles but it doesn't work if only the fix from the above commit
is applied. As OpenStructure implements all functionality from DSSP
(secondary structure and solvent accessibility), the binding to an
external program has been deprecated. dssp.AssignDSSP still exists,
but mimics the old behaviour using OpenStructure internal implementations.

Compilation was broken for years when IMG has been disabled.
IMG is now always built.