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"