Issue #1584: Provide options to override default search paths for Tcl and Tk
when building _tkinter.  configure has two new options; if used, both must
be specified:

  ./configure \
      --with-tcltk-includes="-I/opt/local/include" \
      --with-tcltk-libs="-L/opt/local/lib -ltcl8.5 -ltk8.5"

In addition, the options can be overridden with make:

   make \
       TCLTK_INCLUDES="-I/opt/local/include" \
       TCLTK_LIBS="-L/opt/local/lib -ltcl8.6 -ltk8.6"
diff --git a/configure b/configure
index 68e4784..f865054 100755
--- a/configure
+++ b/configure
@@ -644,6 +644,8 @@
 USE_THREAD_MODULE
 SIGNAL_OBJS
 USE_SIGNAL_MODULE
+TCLTK_LIBS
+TCLTK_INCLUDES
 LIBFFI_INCLUDEDIR
 PKG_CONFIG
 SHLIBS
@@ -786,6 +788,8 @@
 with_libs
 with_system_expat
 with_system_ffi
+with_tcltk_includes
+with_tcltk_libs
 with_dbmliborder
 with_signal_module
 with_dec_threads
@@ -1459,6 +1463,10 @@
   --with-system-expat     build pyexpat module using an installed expat
                           library
   --with-system-ffi       build _ctypes module using an installed ffi library
+  --with-tcltk-includes='-I...'
+                          override search for Tcl and Tk include files
+  --with-tcltk-libs='-L...'
+                          override search for Tcl and Tk libs
   --with-dbmliborder=db1:db2:...
                           order to check db backends for dbm. Valid value is a
                           colon separated string with the backend names
@@ -8954,6 +8962,46 @@
 { $as_echo "$as_me:${as_lineno-$LINENO}: result: $with_system_ffi" >&5
 $as_echo "$with_system_ffi" >&6; }
 
+# Check for --with-tcltk-includes=path and --with-tcltk-libs=path
+
+
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for --with-tcltk-includes" >&5
+$as_echo_n "checking for --with-tcltk-includes... " >&6; }
+
+# Check whether --with-tcltk-includes was given.
+if test "${with_tcltk_includes+set}" = set; then :
+  withval=$with_tcltk_includes;
+else
+  with_tcltk_includes="default"
+fi
+
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $with_tcltk_includes" >&5
+$as_echo "$with_tcltk_includes" >&6; }
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for --with-tcltk-libs" >&5
+$as_echo_n "checking for --with-tcltk-libs... " >&6; }
+
+# Check whether --with-tcltk-libs was given.
+if test "${with_tcltk_libs+set}" = set; then :
+  withval=$with_tcltk_libs;
+else
+  with_tcltk_libs="default"
+fi
+
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $with_tcltk_libs" >&5
+$as_echo "$with_tcltk_libs" >&6; }
+if test "x$with_tcltk_includes" = xdefault || test "x$with_tcltk_libs" = xdefault
+then
+  if test "x$with_tcltk_includes" != "x$with_tcltk_libs"
+  then
+    as_fn_error $? "use both --with-tcltk-includes='...' and --with-tcltk-libs='...' or neither" "$LINENO" 5
+  fi
+  TCLTK_INCLUDES=""
+  TCLTK_LIBS=""
+else
+  TCLTK_INCLUDES="$with_tcltk_includes"
+  TCLTK_LIBS="$with_tcltk_libs"
+fi
+
 # Check for --with-dbmliborder
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for --with-dbmliborder" >&5
 $as_echo_n "checking for --with-dbmliborder... " >&6; }