gitk: Check that we are running on at least Tcl/Tk 8.4

This checks that we have Tcl/Tk 8.4 or later, and puts up an error
message in a window and quits if not.

This was prompted by a patch submitted by Steffen Prohaska, but is
done a bit differently (this uses package require rather than
looking at [info tclversion], and uses show_error to display the
error rather than printing it to stderr).
Signed-off-by: NPaul Mackerras <paulus@samba.org>

gitk

@@ -7838,6 +7838,13 @@ proc tcl_encoding {enc} {
     return {}
 }
 
+# First check that Tcl/Tk is recent enough
+if {[catch {package require Tk 8.4} err]} {
+    show_error {} . "Sorry, gitk cannot run with this version of Tcl/Tk.\n\
+		     Gitk requires at least Tcl/Tk 8.4."
+    exit 1
+}
+
 # defaults...
 set datemode 0
 set diffopts "-U 5 -p"