Fix clang-cl warning flags: use /W4 instead of /Wall

This PR fixes the warning flags when building with **clang-cl**.

Previously, the CMake configuration unconditionally added `-Wall -Wextra` for Clang.  
However, according to the [clang-cl documentation](https://clang.llvm.org/docs/UsersManual.html#clang-cl), `/Wall` in clang-cl is mapped to `-Weverything`, which enables **all possible warnings**, far more than MSVC’s `/Wall`. This results in a flood of warnings that are not intended.

The original intent was to match MSVC `/W4` (≈ `-Wall -Wextra` in GCC/Clang).

Author: SquallATF

CMakeLists.txt

@@ -122,7 +122,13 @@ elseif ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
   if(CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.3.0)
     message(FATAL_ERROR "llvm/clang 3.3 or higher required!")
   endif()
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra")
+  if ("${CMAKE_CXX_COMPILER_FRONTEND_VARIANT}" STREQUAL "GNU")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra")
+  elseif("${CMAKE_CXX_COMPILER_FRONTEND_VARIANT}" STREQUAL "MSVC")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /W4")
+    add_definitions(-DUNICODE -DNOMINMAX)
+    set(CMAKE_DEBUG_POSTFIX _d)
+  endif()
 elseif ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")
   if(MSVC_VERSION LESS 1900)
     message(FATAL_ERROR "msvc 2015 or higher required!")