Allow build to link to an already installed liblsl version

[WillBux]

Build will first find a liblsl via the LIBLSL_LIB_DIR environment variable.

Additionally, the build can automatically find the library on MacOS if installed with homebrew.

Fallback to building the library from source, as is done currently.

I wrote this pr as for some reason liblsl cannot be compiled on my system via CMake, and this was easier than fixing the liblsl build.

[cboulay]

Hi, I'm not involved in managing liblsl-rust but I am involved in managing the upstream liblsl, especially the CI/CD. Can you give me more information as to why you can't use the framework on MacOS? I'm relatively new to building frameworks over dylibs so I'm trying to learn what the limitations are.
Thanks!

[WillBux]

I didn't mean for the last two commits to be added to this PR; however, linking the framework is seemingly a lot more complex than the static library which I am currently building. The framework requires setting some rpaths which I was only able to get working in the local tests but once I imported the lsl-rust library into my project it all fell apart. I am by no means an expert either, but setting LSL_FRAMEWORK=OFF just made everything work as before.