[RDY] 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.
Is it necessary to add `CC=` and `LD=` now?  Also, for my education, what's the advantage of using `build` vs. `install`?  It seems like the `VAR=VALUE` are global luarocks options, so wouldn't it work with `install` as well?