Add 'const' qualifier to some cmCommand members

Use const_cast for the special case in cmFindBase where
GetFullDocumentation calls GenerateDocumentation.

Source/cmExecuteProcessCommand.h

@@ -41,18 +41,18 @@ public:
   /**
    * The name of the command as specified in CMakeList.txt.
    */
-  virtual const char* GetName()
+  virtual const char* GetName() const
     {return "execute_process";}
 
   /**
    * This determines if the command is invoked when in script mode.
    */
-  virtual bool IsScriptable() { return true; }
+  virtual bool IsScriptable() const { return true; }
 
   /**
    * Succinct documentation.
    */
-  virtual const char* GetTerseDocumentation()
+  virtual const char* GetTerseDocumentation() const
     {
     return "Execute one or more child processes.";
     }
@@ -60,7 +60,7 @@ public:
   /**
    * More documentation.
    */
-  virtual const char* GetFullDocumentation()
+  virtual const char* GetFullDocumentation() const
     {
     return
       "  execute_process(COMMAND <cmd1> [args1...]]\n"