ENH: Remove SKIP_RULE_DEPENDS option from add_custom_command()

- Option was recently added but never released.
  - Custom commands no longer depend on build.make so we do
    not need the option.
  - Rule hashes now take care of rebuilding when rules change
    so the dependency is not needed.

Source/cmCustomCommand.cxx

@@ -22,7 +22,6 @@ cmCustomCommand::cmCustomCommand()
   this->HaveComment = false;
   this->EscapeOldStyle = true;
   this->EscapeAllowMakeVars = false;
-  this->SkipRuleDepends = false;
 }
 
 //----------------------------------------------------------------------------
@@ -51,9 +50,10 @@ cmCustomCommand::cmCustomCommand(const std::vector<std::string>& outputs,
   Comment(comment?comment:""),
   WorkingDirectory(workingDirectory?workingDirectory:""),
   EscapeAllowMakeVars(false),
-  EscapeOldStyle(true),
-  SkipRuleDepends(false)
+  EscapeOldStyle(true)
 {
+  this->EscapeOldStyle = true;
+  this->EscapeAllowMakeVars = false;
 }
 
 //----------------------------------------------------------------------------
@@ -135,18 +135,6 @@ void cmCustomCommand::SetEscapeAllowMakeVars(bool b)
   this->EscapeAllowMakeVars = b;
 }
 
-//----------------------------------------------------------------------------
-bool cmCustomCommand::GetSkipRuleDepends() const
-{
-  return this->SkipRuleDepends;
-}
-
-//----------------------------------------------------------------------------
-void cmCustomCommand::SetSkipRuleDepends(bool b)
-{
-  this->SkipRuleDepends = b;
-}
-
 //----------------------------------------------------------------------------
 cmCustomCommand::ImplicitDependsList const&
 cmCustomCommand::GetImplicitDepends() const