Where do I get the "cmake_toolchain_clangcl*" commands needed to cross-compile PuTTY for Windows?

Question

I'm trying to cross-compile PuTTY for Windows with Clang on Linux. When I look at PuTTY's Buildscr file, it sets CMAKE_TOOLCHAIN_FILE to the output of cmake_toolchain_clangcl32, cmake_toolchain_clangcl64, cmake_toolchain_clangcl_a32, or cmake_toolchain_clangcl_a64, depending on which architecture it's compiling for (e.g., cmake ../.. -DCMAKE_TOOLCHAIN_FILE=$(cmake_toolchain_clangcl64) -DCMAKE_BUILD_TYPE=Release -DCMAKE_MSVC_RUNTIME_LIBRARY=MultiThreaded -DPUTTY_LINK_MAPS=ON -DCMAKE_C_FLAGS_RELEASE="/MT /O2" -DSTRICT=ON). But I can't find any other reference to this command anywhere. I don't see it anywhere else in PuTTY's Git repo, bob (the program that interprets Buildscr)'s Git repo, or in any distro's package manager. What are these commands, and where am I supposed to get them from?