[libjpeg-turbo] enable jpeg_mem_src/dest

[attah]

You'll need this for any significant update to Poppler (21.12.0 and onwards).
Sure, this is a bit iffy library-versioning-wise, but i can't see that it actually breaks anything and all the cool kids are doing it.
But then again, C/C++ is not my day job.

[Thaodan]

Please explain the change better and rather implicitly enabling it, enable it explicitly.

This way it is reproducible and it breaks once the option is removed.

[attah]

What this does is make the source/destination managers (i.e. io handlers) that work with in-memory data exist at all, along side with the previous only option that handles FILE*.

See also: https://github.com/sailfishos-mirror/libjpeg-turbo/blob/a219fd13b086d9af53de37a5e1bfec985cda60fc/README.md#in-memory-sourcedestination-managers

I'm not sure i follow, there is nothing that will break if a define no longer does anything or exists inside the code, so -DWITH_MEM_SRCDST=1 would be pointless.

[Thaodan]

What this does is make the source/destination managers (i.e. io handlers) that work with in-memory data exist at all, along side with the previous only option that handles FILE*.

Ok that sounds good, as the api isn't harbour whitelist abi breaks shouldn't matter, if we decide to preserve in abi in this case need to offer both versions.

I'm not sure i follow, there is nothing that will break if a define no longer does anything or exists inside the code, so -DWITH_MEM_SRCDST=1 would be pointless.

Other build systems usually exit if you specific invalid options, e.g if upstream removes or renames it something happen.
But I get it now.

[pvuorela]

Needs now rebasing.

Otherwise seems good to me. Since nothing promised for this library, better to avoid sticking to old ABI.