Add 'const' qualifier to some cmCommand members

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

Source/cmOutputRequiredFilesCommand.h

@@ -40,12 +40,12 @@ public:
   /**
    * The name of the command as specified in CMakeList.txt.
    */
-  virtual const char* GetName() { return "output_required_files";}
+  virtual const char* GetName() const { return "output_required_files";}
 
   /**
    * Succinct documentation.
    */
-  virtual const char* GetTerseDocumentation() 
+  virtual const char* GetTerseDocumentation() const
     {
     return "Deprecated.  Approximate C preprocessor dependency scanning.";
     }
@@ -53,7 +53,7 @@ public:
   /**
    * More documentation.
    */
-  virtual const char* GetFullDocumentation()
+  virtual const char* GetFullDocumentation() const
     {
     return
       "This command exists only because ancient CMake versions provided it.  "
@@ -67,7 +67,7 @@ public:
     }
 
   /** This command is kept for compatibility with older CMake versions. */
-  virtual bool IsDiscouraged()
+  virtual bool IsDiscouraged() const
     {
     return true;
     }