Minor fixes to enable compiling against TCL8.5

Define USE_NON_CONST so that the newer TCL headers define functions in the
same way as pre-8.4 TCL headers.  It will be a good idea to change to
USE_COMPAT_CONST instead at some point.

Also use ckfree() instead of free() to free an allocation returned by TCL.

source/fset.c

@@ -11,6 +11,7 @@ CVS_REVISION(fset_c)
 #include "misc.h"
 #include "list.h"
 #include "vars.h"
+#include "tcl_bx.h"
 #define MAIN_SOURCE
 #include "modval.h"
 

source/tcl_public.c

@@ -1435,7 +1435,7 @@ struct timeval now1;
 			{
 				for (i=0; i<argc; i++) 
 					Tcl_DStringAppendElement(&ds,argv[i]);
-				free(argv);
+				ckfree((char *)argv);
 				code=Tcl_Eval(tcl_interp,Tcl_DStringValue(&ds));
 				/* code=Tcl_Eval(tcl_interp,mark->cmd); */
 				Tcl_DStringFree(&ds);