Add 'const' qualifier to some cmCommand members

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

Source/cmWriteFileCommand.h

@@ -39,17 +39,17 @@ public:
   /**
    * This determines if the command is invoked when in script mode.
    */
-  virtual bool IsScriptable() { return true; }
+  virtual bool IsScriptable() const { return true; }
 
   /**
    * The name of the command as specified in CMakeList.txt.
    */
-  virtual const char* GetName() { return "write_file";}
+  virtual const char* GetName() const { return "write_file";}
 
   /**
    * Succinct documentation.
    */
-  virtual const char* GetTerseDocumentation() 
+  virtual const char* GetTerseDocumentation() const
     {
     return "Deprecated. Use the file(WRITE ) command instead.";
     }
@@ -57,7 +57,7 @@ public:
   /**
    * More documentation.
    */
-  virtual const char* GetFullDocumentation()
+  virtual const char* GetFullDocumentation() const
     {
     return
       "  write_file(filename \"message to write\"... [APPEND])\n"
@@ -73,7 +73,7 @@ public:
     }
   
   /** This command is kept for compatibility with older CMake versions. */
-  virtual bool IsDiscouraged()
+  virtual bool IsDiscouraged() const
     {
     return true;
     }