(#7290) Define boost::system as header-only component

Starting v1.69.0 Boost.System is header-only. A stub library is still
built for compatibility, but linking to it is no longer necessary.
https://www.boost.org/doc/libs/1_75_0/libs/system/doc/html/system.html#changes_in_boost_1_69

recipes/boost/all/conanfile.py

@@ -1513,6 +1513,13 @@ def filter_transform_module_libraries(names):
                 if set(module_libraries).difference(all_detected_libraries):
                     incomplete_components.append(module)
 
+                # Starting v1.69.0 Boost.System is header-only. A stub library is
+                # still built for compatibility, but linking to it is no longer
+                # necessary.
+                # https://www.boost.org/doc/libs/1_75_0/libs/system/doc/html/system.html#changes_in_boost_1_69
+                if module == "system":
+                    module_libraries = []
+
                 self.cpp_info.components[module].libs = module_libraries
 
                 self.cpp_info.components[module].requires = self._dependencies["dependencies"][module] + ["_libboost"]