Fix pretty printing of configure output for rust checks

Right now we print:

[…]
checking for cargo... cargo
checking rust crate dependencies... checking rust version... checking for library containing socket... none required
[…]

I think we're supposed to use AC_MSG_RESULT to print "yes" if the rust crate dependency checks go okay. After the "checking rust version" we should probably also print something indicating that the version was acceptible (I'd personally go for printing the version string, since it doesn't hurt to have that in the build logs).