add c compiler

Author: pifopi

.github/workflows/cpp-ci-serial-programs.yml

@@ -23,7 +23,7 @@ jobs:
 
           - os: 'ubuntu-24.04'
             compiler: 'clang'
-            cmake_additional_param: '-DCMAKE_CXX_COMPILER=clang++'
+            cmake_additional_param: '-DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++'
 
         exclude:
           - os: 'macos-13'