fix: Ensure that test suite passes with system-provided SQLite on MacOS

[paleolimbot]

I opened up the C drivers and ran the tests and got a failing test: when the driver compiles against system SQLite on MacOS, extension loading is not allowed (the symbols aren't even provided nor defined in the header). I added a define to make sure the R package could build on MacOS, but the test needs it too.

I also added come CMake to check this...I also tried check_sybmol_exists but this seems to require setting CMAKE_REQUIRED_LIBRARIES/includes which seems like it might have some global impact that we don't want. Checking for the text in the header does the trick for me (and is maybe less likely to result in accidentally building the library without extension support).

The other failing test I saw before seems to have been cleared up by the SQLite driver refactor!

The .cache in the .gitignore is because clangd seems to put a lot of files there.

[lidavidm]

It looks like CI is passing here?

[paleolimbot]

I forgot to circle back here...I'm pretty sure all of our MacOS CI checks don't use system libsqlite. This is pretty much only an issue if you're a contributor to the SQLite driver who isn't working in a conda environment (which is probably just me, so pretty low priority).