Preferring c over rust

[John Klos <john%ziaspace.com@localhost>]

I just noticed this failure:

=> Verifying reinstall for ../../lang/rust
=> Bootstrap dependency digest>=20010302: found digest-20190127
ERROR: This package is not available for these platforms: 
NetBSD-8.99.42-alpha.
ERROR: This package has set PKG_SKIP_REASON:
ERROR: rust-1.34.2 is not available for NetBSD-8.99.42-alpha
*** Error code 1

Stop.
make[1]: stopped in /usr/pkgsrc/lang/rust
*** Error code 1

Stop.
make: stopped in /usr/pkgsrc/graphics/librsvg
*** 'make replace' failed for package librsvg.
*** Please read the errors listed above, fix the problem,
*** then re-run pkg_rolling-replace to continue.


The commit note says:

This package now uses rust. The user-settable variable LIBRSVG_USE_RUST
allows switching between this and the (obsolete) plain-C version
in librsvg-c. It defaults to "yes" on platforms that lang/rust currently
supports.


As projects move to non-portable languages, should we consider keeping the 
latest / last version which is still portable? In this case, I think it 
just needs to be fixed to prefer c, but in general I wonder what others' 
thoughts are about this kind of thing.

John