[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.
Only adding `CC` didn't use `-m32` during linking, so I added `LD` as well. I changed it do `build` to make sure the dependencies are compiled, not installed from a downloaded binary package.