On Travis, fetching file creation time fails. Because codegen tests
depended on being able to retrieve this time, the tests themselves would
fail. Instead of failing outright, this commit makes an effort to find
_some_ time to associate with a file's creation, even if the time is
suboptimal.
In this commit, Rocket's codegen tests now search for the correct
library to depend on, independent of compiletest. This solution should
be robust against platform differences.