RFR: 8013785 Respect EXTRA_CFLAGS on windows

[Staffan Larsen]

EXTRA_CFLAGS can be used on all platforms except windows to add extra compiler arguments to the compile. This fix improves the  windows build to also support this.

webrev: http://cr.openjdk.java.net/~sla/8013785/webrev.00/

Thanks,
/Staffan