Portable UNUSED parameter macro used on function signature for C and C++

I'm interested in creating a macro for eliminating the unused variable warning.

This question describes a way to suppress the unused parameter warning by writing a macro inside the function code:

Universally compiler independent way of implementing an UNUSED macro in C/C++

But I'm interested in a macro that can be used in the function signature:

void callback(int UNUSED(some_useless_stuff)) {}

This is what I dug out using Google (source)

#ifdef UNUSED
#elif defined(__GNUC__)
# define UNUSED(x) UNUSED_ ## x __attribute__((unused))
#elif defined(__LCLINT__)
# define UNUSED(x) /*@unused@*/ x
#elif defined(__cplusplus)
# define UNUSED(x)
#else
# define UNUSED(x) x
#endif

Can this be further expanded for other compilers?

Edit: For those who can't understand how tagging works: I want a solution for both C and C++. That is why this question is tagged both C and C++ and that is why a C++ only solution is not acceptable.

The way I do it is like this:

#define UNUSED(x) (void)(x)
void foo(const int i) {
    UNUSED(i);
}

I've not had a problem with that in Visual Studio, Intel, gcc and clang.

The other option is to just comment out the parameter:

void foo(const int /*i*/) {
  // When we need to use `i` we can just uncomment it.
}

Just one small thing, better using __attribute__((__unused__)) as __attribute__((unused)), because unused could be somewhere defined as macro, personally I had a few issues with this situation.

But the trick I'm using is, which I found more readable is:

#define UNUSED(x) (void)x;

It works however only for the variables, and arguments of the methods, but not for the function itself.

After testing and following the comments, the original version mentioned in the question turned out to be good enough.

Using: #define UNUSED(x) __pragma(warning(suppress:4100)) x (mentioned in comments), might be necessary for compiling C on MSVC, but that's such a weird combination, that I didn't include it in the end.

Across many compilers I have used the following, excluding support for lint.

#if (__GNUC__ > 2) || (__GNUC__ == 2 && __GNUC_MINOR__ > 4)
#       define PGM_GNUC_UNUSED         __attribute__((__unused__))
#else
#       define PGM_GNUC_UNUSED
#endif

Tested compilers: GCC, Clang, EKOPath, Intel C Compiler / Composer XE, MinGW32 on Cygwin / Linux / MSYS, MinGW-w64 on Cygwin / Linux, Sun ONE Studio / Oracle Solaris Studio, Visual Studio 2008 / 2010.

Example usage:

pgm_tsc_init (
        PGM_GNUC_UNUSED pgm_error_t**   error
        )
{
...
}

PGM is the standard prefix for this C based project. GNUC is the convention from GLib for this attribute.

I think one compile warns about __attribute__ in certain circumstances but certainly no error.