cmake: stop exporting compat functions

[vszakats]

Attempt to avoid EXTRA_EXPORT and LIBTLS_EXTRA_EXPORT, by doing something
similar to what autotools builds do already.

Fixes #969

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Locally I could only test a 3.8.2-based iteration and one that didn't tackle LIBTLS_EXTRA_EXPORT. CI should tell more.

[botovq]

Amazing. Many thanks!

I have come up with a subset of this diff, but haven't gotten quite as far. All the changes make sense. I wonder if we can use a relatively recent API that will likely be needed everywhere such as recallocarray in place of empty.c.

[vszakats]

I wonder if we can use a relatively recent API that will likely be needed everywhere such as recallocarray in place of empty.c.

That would make sense. (Another option is making the compat lib names a variable, then leaving them empty and omitting these targets altogether when empty. Though this has more moving parts so I'd only do it if the compat functions are expected to disappear mid term.)

[busterb]

Note: skipping this PR for stable