GCC attribute alias should not trigger warning

We spuriously warned about an extern symbol with value, where it was
only our handling of the "alias" attribute that introduced that value in
the first place. While at it, make that warning more useful.

Author: tautschnig

regression/ansi-c/gcc_attributes12/test.desc

@@ -1,8 +1,9 @@
 CORE gcc-only
 main.c
-
+-Wall
 ^EXIT=0$
 ^SIGNAL=0$
 --
+'extern' symbol 'my_ids_table' should not have an initializer
 ^warning: ignoring
 ^CONVERSION ERROR$

src/ansi-c/c_typecheck_base.cpp

@@ -71,13 +71,14 @@ void c_typecheck_baset::typecheck_symbol(symbolt &symbol)
     new_name=root_name;
     symbol.is_static_lifetime=true;
 
-    if(symbol.value.is_not_nil())
+    if(symbol.value.is_not_nil() && !symbol.is_macro)
     {
       // According to the C standard this should be an error, but at least some
       // versions of Visual Studio insist to use this in their C library, and
       // GCC just warns as well.
       warning().source_location = symbol.value.find_source_location();
-      warning() << "`extern' symbol should not have an initializer" << eom;
+      warning() << "'extern' symbol '" << new_name
+                << "' should not have an initializer" << eom;
     }
   }
   else if(!is_function && symbol.value.id()==ID_code)