build: Make libSDL2 optional and only link orblcd against it

[ALTracer]

While building this suite on Windows 10 under Msys2 UCRT64 I encountered that I have to extract libSDL2.dll and carry it in the "redist" archive to be able to run any of resulting binaries on other machines. Also nothing compiled until I installed said libSDL2 via pacman.

Looking at #143 I wrote a small patch, knowing very little meson, which decouples libSDL2 from command-line tools, and that hypothetically allows me to omit both orblcd and libSDL2.dll and have smaller redist archives (we don't use orblcd in our development workflows).

This is a follow-up to the discussion from 10 days ago.

$ objdump -p build-gcc12/orblcd | grep NEEDED
  NEEDED               libasan.so.8
  NEEDED               liborb.so.2.1.0
  NEEDED               libSDL2-2.0.so.0
  NEEDED               libc.so.6
$ objdump -p build-gcc12/orbuculum | grep NEEDED
  NEEDED               libasan.so.8
  NEEDED               liborb.so.2.1.0
  NEEDED               libusb-1.0.so.0
  NEEDED               libc.so.6
$ objdump -p build-gcc12/orbtop | grep NEEDED
  NEEDED               libasan.so.8
  NEEDED               liborb.so.2.1.0
  NEEDED               libc.so.6

Ignore the libasan, what's important is only needed libraries are linked for each respective tool. I have yet to test this on Windows, so please allow some time before merging.

If ucrt64 actually links more than needed, then I may want to decouple libzmq in a similar way (we don't use orbzmq either). I'm open for discussion whether my meson-fu allows me to tackle that here.

[zyp]

Looks good to me, and handling libzmq in the same way should be fine.