Implement the `--enable-versioned-symbols` feature available in
`./configure` for CMake.
Enable with `-DCURL_LIBCURL_VERSIONED_SYMBOLS=ON`. Customize the version
prefix with `-DCURL_LIBCURL_VERSIONED_SYMBOLS_PREFIX=MYPREFIX_`. By
default the prefix matches what autotools uses.
When enabled, the feature is detected and a warning shown if
unavailable. (E.g. on Apple and Windows, it isn't.)
Included `HIDDEN {};` to match autotools, though I don't know if it's
necessary, useful or making any difference.
Differences from the autotools implementation:
- soversion is dynamic instead of hard-coded.
- omits referencing non-curl symbols.
- allows prefix/flavour override.
- more universal feature detection.
- doesn't rely on the in-repo `lib/libcurl.vers.in` file.
Also:
- add mbedTLS and BearSSL versioned symbol prefix support to autotools.
- enable this option in an old-linux job.
Follow-up to 7cc2e8b349Fixes#14349Closes#14378
7b14449790