Fix link error on linux (64-bit systems, except debian) caused by missing library search directory

[aabizri]

Right now, if we build liblsl-rust on a non-debian linux distro (e.g fedora, nixOS) targeting a 64-bit system, we get a linking error at the end:

error: could not find native static library `lsl`, perhaps an -L flag is missing?

This is because, as shown in this SO topic, CMake projects relying on GNUInstallDirs to configure the directories (like liblsl), have a quirk for this category of targets.

I've added a check in build.rs, which detects if we're targeting a 64-bit linux system, and adds the lib64 directory to the library search directories accordingly. This still works for debian-based targets as the lib search directory is still there.

[berkgercek]

@chkothe would it be possible to merge this fix? Without it currently the lsl crate will not compile on any Linux systems other than Ubuntu. Thank you @aabizri for the work.