Warn about .C and .H files (#8249)

mesonbuild/mesonlib/universal.py

@@ -348,8 +348,15 @@ class FileMode:
             perms |= stat.S_ISVTX
         return perms
 
+dot_C_dot_H_warning = """You are using .C or .H files in your project. This is deprecated.
+         Currently, Meson treats this as C code, but this
+            might change in the future, breaking your build.
+         You code also might be already broken on gcc and clang.
+         See https://github.com/mesonbuild/meson/pull/8239 for the discussions."""
 class File:
     def __init__(self, is_built: bool, subdir: str, fname: str):
+        if fname.endswith(".C") or fname.endswith(".H"):
+            mlog.warning(dot_C_dot_H_warning, once=True)
         self.is_built = is_built
         self.subdir = subdir
         self.fname = fname