guile-gnome: fix build by building gwrap with guile-2.0

Apparently gwrap will not compile with guile-2.2 [1], even though the
news for version 1.9.15 says it "allows" Guile 2.2 [2]:

> it will _not_ compile using 2.2

Furthermore, it seems like it isn't being developed anymore either [1]:

> Also note that g-wrap itself is not being further developed anymore,
> it is recommended for new projects to use Guile's dynamic FFI.

Also, guile-gnome-2.16.5 is apparently compatible with guile-2.2 [3],
but I'm not sure how they built it with guile-2.2 because gwrap 1.9.15
(latest release) apparently doesn't build with guile-2.2. (And certainly
when I try to build gwrap 1.9.15 with guile-2.2 it doesn't work. Maybe
it can be made to work with certain compile flags, but I haven't pursued
that further due to [1] anyway.) This is why guile-gnome is still on
2.16.4 here. Because, although 2.16.5 can still (apparently) build with
guile-2.0.14, guile_2_0 is only at guile-2.0.13.

So to update guile-gnome to 2.16.5, guile_2_0 would first have to be
updated to 2.0.14.

[1]: http://lists.nongnu.org/archive/html/g-wrap-dev/2016-08/msg00001.html
[2]: http://www.nongnu.org/g-wrap/news.html
[3]: https://www.gnu.org/software/guile-gnome/news.html