Change license of filenames.h to LGPL

[Eli Zaretskii]

> Cc: gcc-patches@gcc.gnu.org, sezeroz@gmail.com
> From: Jeff Law <law@redhat.com>
> Date: Tue, 27 Sep 2016 12:36:11 -0600
> 
> On 09/27/2016 11:52 AM, DJ Delorie wrote:
> >
> > Most of the files in include/ are GPL, not LGPL.  Why is this one
> > different?
> Right.  ANd it's not like this file inserts anything of significance 
> into the resulting object code.  I'd really like to see more rationale 
> behind the request for a license change.

See my other message.

In the original request I reported that this change was already
approved by Richard Stallman, who asked these same questions.