Merge #1647: cmake: Adjust diagnostic flags for clang-cl

4c50d73 ci: Add new "Windows (clang-cl)" job (Hennadii Stepanov)
84c0bd1 cmake: Adjust diagnostic flags for clang-cl (Hennadii Stepanov)

Pull request description:

  When building with `clang-cl` on Windows, the output is cluttered with warning messages because compiler diagnostic flags are not applied correctly:
  ```
  > cmake -B build -G Ninja -DCMAKE_C_COMPILER="C:\Users\hebasto\Downloads\clang+llvm-18.1.8-x86_64-pc-windows-msvc\bin\clang-cl.exe"
  > cmake --build build
  [1/16] Building C object src\CMakeFiles\bench.dir\bench.c.obj
  In file included from C:\Users\hebasto\secp256k1\src\bench.c:11:
  C:\Users\hebasto\secp256k1\src\util.h(34,13): warning: unused function 'print_buf_plain' [-Wunused-function]
     34 | static void print_buf_plain(const unsigned char *buf, size_t len) {
        |             ^~~~~~~~~~~~~~~
  1 warning generated.
  [2/16] Building C object src\CMakeFiles\secp256k1_precomputed.dir\precomputed_ecmult_gen.c.obj
  In file included from C:\Users\hebasto\secp256k1\src\precomputed_ecmult_gen.c:3:
  In file included from C:\Users\hebasto\secp256k1\src\group.h:10:
  In file included from C:\Users\hebasto\secp256k1\src\field.h:10:
  C:\Users\hebasto\secp256k1\src\util.h(34,13): warning: unused function 'print_buf_plain' [-Wunused-function]
     34 | static void print_buf_plain(const unsigned char *buf, size_t len) {
        |             ^~~~~~~~~~~~~~~
  ```

  This PR resolves this issue.

  ---

  **Additional note for reviewers:** The VS builtin clang can also be used assuming that the following VS components are installed:

  ![photo_2024-12-12_12-38-17](https://github.com/user-attachments/assets/c69bafcc-3aa2-4a72-a162-071c593d1c4a)

  The user can generate a build system on Windows as follows:
  - Using the default "Visual Studio" generator:
  ```
  cmake -B build -T ClangCL
  ```
  - Using the "Ninja" generator:
  ```
  cmake -B build -G Ninja -DCMAKE_C_COMPILER=clang-cl
  ```

  ---

  Required for downstream projects which aim to build with `clang-cl` (see bitcoin/bitcoin#31456).

ACKs for top commit:
  real-or-random:
    utACK 4c50d73

Tree-SHA512: 439eb53afd7be65d538cd569f3d095f58325bd26ffc5014ca5f94320689a45b20c9a5a963170578214a20fd3233ec15ef6ab75ab96ce3a4314c282b1b6229ca1

Author: real-or-random

.github/workflows/ci.yml

@@ -746,6 +746,8 @@ jobs:
             cpp_flags: '/DSECP256K1_MSVC_MULH_TEST_OVERRIDE'
           - job_name: 'x86 (MSVC): Windows (VS 2022)'
             cmake_options: '-A Win32'
+          - job_name: 'x64 (MSVC): Windows (clang-cl)'
+            cmake_options: '-T ClangCL'
 
     steps:
       - name: Checkout

CMakeLists.txt

@@ -242,17 +242,21 @@ endif()
 
 include(TryAppendCFlags)
 if(MSVC)
-  # Keep the following commands ordered lexicographically.
+  # For both cl and clang-cl compilers.
   try_append_c_flags(/W3) # Production quality warning level.
+  # Eliminate deprecation warnings for the older, less secure functions.
+  add_compile_definitions(_CRT_SECURE_NO_WARNINGS)
+else()
+  try_append_c_flags(-Wall) # GCC >= 2.95 and probably many other compilers.
+endif()  
+if(CMAKE_C_COMPILER_ID STREQUAL "MSVC")
+  # Keep the following commands ordered lexicographically.
   try_append_c_flags(/wd4146) # Disable warning C4146 "unary minus operator applied to unsigned type, result still unsigned".
   try_append_c_flags(/wd4244) # Disable warning C4244 "'conversion' conversion from 'type1' to 'type2', possible loss of data".
   try_append_c_flags(/wd4267) # Disable warning C4267 "'var' : conversion from 'size_t' to 'type', possible loss of data".
-  # Eliminate deprecation warnings for the older, less secure functions.
-  add_compile_definitions(_CRT_SECURE_NO_WARNINGS)
 else()
   # Keep the following commands ordered lexicographically.
   try_append_c_flags(-pedantic)
-  try_append_c_flags(-Wall) # GCC >= 2.95 and probably many other compilers.
   try_append_c_flags(-Wcast-align) # GCC >= 2.95.
   try_append_c_flags(-Wcast-align=strict) # GCC >= 8.0.
   try_append_c_flags(-Wconditional-uninitialized) # Clang >= 3.0 only.