[client/common] windows: fix native clang-cl build

client/cmake/toolchains/windows-clangcl.cmake

@@ -0,0 +1,28 @@
+if(NOT DEFINED ENV{VSCMD_VER} OR
+    NOT "$ENV{VSCMD_ARG_TGT_ARCH}" MATCHES "^(x64|amd64)$" OR
+    NOT "$ENV{VSCMD_ARG_HOST_ARCH}" MATCHES "^(x64|amd64)$")
+  message(FATAL_ERROR
+    "The windows-clangcl preset requires the x64 Native Tools environment; "
+    "open an x64 Native Tools command prompt and configure again")
+endif()
+
+find_program(LG_WINDOWS_CLANG_CL
+  NAMES clang-cl
+  PATHS "$ENV{VCINSTALLDIR}/Tools/Llvm/x64/bin"
+  NO_DEFAULT_PATH
+  REQUIRED)
+execute_process(
+  COMMAND "${LG_WINDOWS_CLANG_CL}" -print-target-triple
+  RESULT_VARIABLE LG_WINDOWS_CLANG_RESULT
+  OUTPUT_VARIABLE LG_WINDOWS_CLANG_TARGET
+  ERROR_VARIABLE  LG_WINDOWS_CLANG_ERROR
+  OUTPUT_STRIP_TRAILING_WHITESPACE)
+if(NOT LG_WINDOWS_CLANG_RESULT EQUAL 0 OR
+    NOT LG_WINDOWS_CLANG_TARGET MATCHES "^x86_64-")
+  message(FATAL_ERROR
+    "The x64 Native Tools environment selected an invalid clang-cl: "
+    "${LG_WINDOWS_CLANG_ERROR}${LG_WINDOWS_CLANG_TARGET}")
+endif()
+
+set(CMAKE_C_COMPILER   "${LG_WINDOWS_CLANG_CL}" CACHE FILEPATH "" FORCE)
+set(CMAKE_CXX_COMPILER "${LG_WINDOWS_CLANG_CL}" CACHE FILEPATH "" FORCE)