CINDEX_LINKAGE int clang_getMacroArgCount(CXCursor C) {
  if (C.kind == CXCursor_MacroDefinition) {
    CXTranslationUnit tu = getCursorTU(C);
    ASTUnit *CXXUnit = static_cast<ASTUnit*>(tu->TUData);
    MacroInfo *info = CXXUnit->getPreprocessor().getMacroInfo(
            const_cast<IdentifierInfo *>(
                cxcursor::getCursorMacroDefinition(C)->getName()));
    return info->getNumArgs();
  } else {
      return -1;
  }
}


Also! don't forget to add the function to libclang.exports