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.ac b/configure.ac
index d922bc7..891d568 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2289,6 +2289,34 @@
 
 AC_MSG_RESULT($with_system_ffi)
 
+# Check for --with-tcltk-includes=path and --with-tcltk-libs=path
+AC_SUBST(TCLTK_INCLUDES)
+AC_SUBST(TCLTK_LIBS)
+AC_MSG_CHECKING(for --with-tcltk-includes)
+AC_ARG_WITH(tcltk-includes,
+            AS_HELP_STRING([--with-tcltk-includes='-I...'], [override search for Tcl and Tk include files]),
+            [],
+            [with_tcltk_includes="default"])
+AC_MSG_RESULT($with_tcltk_includes)
+AC_MSG_CHECKING(for --with-tcltk-libs)
+AC_ARG_WITH(tcltk-libs,
+            AS_HELP_STRING([--with-tcltk-libs='-L...'], [override search for Tcl and Tk libs]),
+            [],
+            [with_tcltk_libs="default"])
+AC_MSG_RESULT($with_tcltk_libs)
+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
+    AC_MSG_ERROR([use both --with-tcltk-includes='...' and --with-tcltk-libs='...' or neither])
+  fi
+  TCLTK_INCLUDES=""
+  TCLTK_LIBS=""
+else
+  TCLTK_INCLUDES="$with_tcltk_includes"
+  TCLTK_LIBS="$with_tcltk_libs"
+fi
+
 # Check for --with-dbmliborder
 AC_MSG_CHECKING(for --with-dbmliborder)
 AC_ARG_WITH(dbmliborder,