Only run check-linkchecker when actually building docs
Only run check-linkchecker when actually building docs

Otherwise the build fails, when running tests but not building docs, e.g.:
https://buildd.debian.org/status/fetch.php?pkg=rustc&arch=ppc64el&ver=1.17.0%2Bdfsg2-3&stamp=1497403375&raw=0

GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust