Add the ALIAS target concept for libraries and executables.

* The ALIAS name must match a validity regex.
* Executables and libraries may be aliased.
* An ALIAS acts immutable. It can not be used as the lhs
  of target_link_libraries or other commands.
* An ALIAS can be used with add_custom_command, add_custom_target,
  and add_test in the same way regular targets can.
* The target of an ALIAS can be retrieved with the ALIASED_TARGET
  target property.
* An ALIAS does not appear in the generated buildsystem. It
  is kept separate from cmMakefile::Targets for that reason.
* A target may have multiple aliases.
* An ALIAS target may not itself have an alias.
* An IMPORTED target may not have an alias.
* An ALIAS may not be exported or imported.

Source/cmTargetPropCommandBase.cxx

@@ -26,6 +26,11 @@ bool cmTargetPropCommandBase
     }
 
   // Lookup the target for which libraries are specified.
+  if (this->Makefile->IsAlias(args[0].c_str()))
+    {
+    this->SetError("can not be used on an ALIAS target.");
+    return false;
+    }
   this->Target =
     this->Makefile->GetCMakeInstance()
     ->GetGlobalGenerator()->FindTarget(0, args[0].c_str());