Using: jruby 1.7.23 (1.9.3p551) 2015-11-24 f496dd5 on Java HotSpot(TM) 64-Bit Server VM 1.7.0_79-b15 +jit [Windows 7-amd64]
A related issue, #1614, was closed on the grounds, that it was finally fixed (for Windows).
At least in JRuby 1.7.23, installed from
https://s3.amazonaws.com/jruby.org/downloads/1.7.23/jruby_windows_x64_1_7_23.exe
the warning still occurs, when I do a
jruby -c FILENAME