Mop-up for setting minimum Tcl version to 8.4.

Commit e2609323 set the minimum Tcl version we support to 8.4, but
I forgot to adjust the documentation to say the same.  Some nosing
around for other consequences found that the configure script could
be simplified slightly as well.

config/tcl.m4

@@ -4,7 +4,7 @@
 
 
 AC_DEFUN([PGAC_PATH_TCLSH],
-[AC_PATH_PROGS(TCLSH, [tclsh tcl tclsh8.6 tclsh86 tclsh8.5 tclsh85 tclsh8.4 tclsh84 tclsh8.3 tclsh83])
+[AC_PATH_PROGS(TCLSH, [tclsh tcl tclsh8.6 tclsh86 tclsh8.5 tclsh85 tclsh8.4 tclsh84])
 if test x"$TCLSH" = x""; then
   AC_MSG_ERROR([Tcl shell not found])
 fi

configure

@@ -14955,7 +14955,7 @@ fi
 
 # Check for Tcl configuration script tclConfig.sh
 if test "$with_tcl" = yes; then
-    for ac_prog in tclsh tcl tclsh8.6 tclsh86 tclsh8.5 tclsh85 tclsh8.4 tclsh84 tclsh8.3 tclsh83
+    for ac_prog in tclsh tcl tclsh8.6 tclsh86 tclsh8.5 tclsh85 tclsh8.4 tclsh84
 do
   # Extract the first word of "$ac_prog", so it can be a program name with args.
 set dummy $ac_prog; ac_word=$2

doc/src/sgml/installation.sgml

@@ -226,9 +226,8 @@ su - postgres
      <para>
       To build the <application>PL/Tcl</application>
       procedural language, you of course need a <productname>Tcl</>
-      installation.  If you are using a pre-8.4 release of
-      <productname>Tcl</>, ensure that it was built without multithreading
-      support.
+      installation.  The minimum required version is
+      <productname>Tcl</productname> 8.4.
      </para>
     </listitem>