[RFC] Automatic dependency rebuilding: Pass CMAKE_C_COMPILER_ARG1 to deps building.

Passes on the `CMAKE_C_COMPILER_ARG1` argument to dependency builds to simplify 32-bit builds. `CMAKE_C_COMPILER_ARG1` is set in `cmake/i386-linux-gnu.toolchain.cmake`.

Prerequisite for neovim/bot-ci#8, but can be merged independently.
> Only adding CC didn't use -m32 during linking (and thus caused an error), so I added LD as well (this was already there in August, not related to the rebase a few days ago).

Okay, that makes sense.

> I changed install to build to make sure the dependencies are compiled, not installed from a downloaded binary package. I think there are no prebuilt binaries for our dependencies, so install fell back to compiling anyway, but I wanted to be explicit.

That sounds good too.  Thank you for the explanation.

> I don't have any experience with luarocks, though, just got all of this from the manpage/internet.

I think at this level, it's most everyone at this point.  It still feels like early days on the luarocks front. :-)

Thanks again!