cross-compilation scaffolding WIP

requires clang/lld or mingw, obviously

just make sure the MSVC_DIRECTORY env variable points to valid Visual Studio Build Tools directory
and do: `cmake -S . -B build -DCMAKE_TOOLCHAIN_FILE=clang_or_mingw_toolchain_file`

cmake/clang-toolchain.cmake

@@ -0,0 +1,39 @@
+set(CMAKE_SYSTEM_NAME Windows)
+
+set(CMAKE_C_COMPILER clang)
+set(CMAKE_CXX_COMPILER clang++)
+set(CMAKE_RC_COMPILER llvm-rc)
+
+set(triple i386-pc-win32-msvc)
+set(CMAKE_C_COMPILER_TARGET ${triple})
+set(CMAKE_CXX_COMPILER_TARGET ${triple})
+set(CMAKE_RC_COMPILER_TARGET ${triple})
+
+set(msvc_dir "$ENV{MSVC_DIRECTORY}")
+if(msvc_dir STREQUAL "")
+	message(FATAL_ERROR "Please set a valid MSVC_DIRECTORY environment variable")
+endif()
+
+# Don't remember which version of Visual Studio Build Tools this is.
+# When updating this, make sure to pin the version so people can get it.
+set(msvc_include
+       "${msvc_dir}/14.34.31933/include"
+       "${msvc_dir}/14.34.31933/ATLMFC/include"
+       "${msvc_dir}/include/10.0.22000.0/ucrt"
+       "${msvc_dir}/include/10.0.22000.0/um"
+       "${msvc_dir}/include/10.0.22000.0/shared"
+       "${msvc_dir}/include/10.0.22000.0/winrt"
+       "${msvc_dir}/include/10.0.22000.0/cppwinrt"
+)
+set(msvc_libraries
+       "${msvc_dir}/14.34.31933/ATLMFC/lib/x86"
+       "${msvc_dir}/14.34.31933/lib/x86"
+       "${msvc_dir}/lib/10.0.22000.0/ucrt/x86"
+       "${msvc_dir}/lib/10.0.22000.0/um/x86"
+)
+
+foreach(LANG C CXX)
+	set(CMAKE_${LANG}_STANDARD_INCLUDE_DIRECTORIES ${msvc_include})
+endforeach()
+
+link_directories(${msvc_libraries})

cmake/mingw-toolchain.cmake

@@ -0,0 +1,11 @@
+set(CMAKE_SYSTEM_NAME Windows)
+set(CMAKE_SYSTEM_PROCESSOR i686)
+
+set(triple ${CMAKE_SYSTEM_PROCESSOR}-w64-mingw32)
+
+set(CMAKE_C_COMPILER ${triple}-gcc)
+set(CMAKE_CXX_COMPILER ${triple}-g++)
+set(CMAKE_RC_COMPILER ${triple}-windres)
+
+set(CMAKE_C_COMPILER_TARGET ${triple})
+set(CMAKE_CXX_COMPILER_TARGET ${triple})