Add baseruby version message

Because `--with-baseruby=/usr/bin/ruby` on macOS is Ruby 2.6,
I was confused why `--with-baseruby` was ignored.

tool/missing-baseruby.bat

@@ -2,4 +2,5 @@
 @echo off
 : "
 echo executable host ruby is required.  use --with-baseruby option.
+echo Note that BASERUBY must be Ruby 2.7.0 or later.
 exit 1