Attempt to resolve SSL_connect missing symbol problem

[arjenpdevries]

This pull request modifies CMakeLists.txt based on comparison with the HTTPFS extension, to solve a building problem reported in issue #89

[Y--]

We're not building the ui_loadable_extension so we should remove this

[arjenpdevries]

Hmmm. That's not true, because of the line above this: build_loadable_extension(${TARGET_NAME} " " ${EXTENSION_SOURCES}).

And if I remove the line for target_link_libraries(ui_extension) my setting of building and running the extension still succeeds.

[arjenpdevries]

So I have additionally tried with both target_link_libraries(ui_extension) and target_link_libraries(ui_loadable_extension) and either in isolation, and the only success is with having at least ui_loadable_extension in the instructions.

As discussed in the review, maybe I did misunderstand that ui could be an extension that was meant to always be linked in; but this now does work with loading the extension as well, i.e., during the build ui is defined as a module only loaded on demand:

-- Extensions linked into DuckDB: [core_functions, parquet, jemalloc]
-- Extensions built but not linked: [httpfs, ui, fts]

Example of running it after building with these changes:

[arjen@apc extension]$ ../duckdb -unsigned
v1.2.1-dev648 cd0d0da9b1
Enter ".help" for usage hints.
D install 'ui/ui.duckdb_extension';
D load ui;
D call start_ui();
┌──────────────────────────────────────┐
│                result                │
│               varchar                │
├──────────────────────────────────────┤
│ UI started at http://localhost:4213/ │
└──────────────────────────────────────┘