[eigen] moving forward with MPL2

[Benoit Jacob <jacob.benoit.1@xxxxxxxxx>]

Hi List,

Can't believe it's been 6 months already!

Anyway, the MPL2 relicensing effort has stalled out. I gather that the
main hesitations about it were:
 1. MPL2 is a new untested license
 2. status of GPL/LGPL compatibility not fully obvious from the license or FAQ

Regarding 1. notice that since then, Mozilla has effectively
relicensed most of its code to MPL2, and all new code should be MPL2,
so there is in fact quite a bit of MPL2 code around now. But still,
not nearly as much as established licenses, of course.

In my original proposal, I said that we should relicense to MPL2-only
instead of tri-licensing, as tri-licensing had proved a weak choice
for Mozilla, as it opened the door to GPL-only back-contributions that
we wouldn't be able to incorporate in the original product.

However, 6 months after, it seems like a good time to ease corners a
bit to make at least something happen, as the statu quo (LGPL/GPL
only) is really not great.

So how about this: relicense to MPL2 + existing LGPL3+/GPL2+ licenses
(i.e. tri-license)? At least this doesn't have a lot of possible
disadvantages compared to the current situation. The above-mentioned
loophope already exists with our present dual-license.

Keeping the existing LGPL+GPL licenses should remove the above
concerns 1. and 2.

Objections?

Benoit