Features for gzip, png, jpeg, ffmpeg

[ExpHP]

Features could be added to require these.

They should somehow use these C API functions to validate that the functionality is present in system libraries:

int lammps_config_has_gzip_support();
int lammps_config_has_png_support();
int lammps_config_has_jpeg_support();
int lammps_config_has_ffmpeg_support();

When activated in an automatic build, they should set the corresponding cmake flags to yes.

I don't think these features should be enabled by default, for the same reasons that exceptions is not on by default. However, unlike for mpi, I believe it is okay that cmake should be allowed to enable these libraries if they are found, even without the feature. (so without the feature, we should do nothing, rather than defining WITH_JPEG=no). This is because they do not require additional effort from us to link them, given that cmake could find them. (whereas linking MPI requires querying mpicxx)

...I think. I could be wrong.

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I have no need for any of this myself, so I'll leave the effort to somebody who does. ;)