Don't treat link-time warnings as errors

We want to use mktemp(3) in the mktemp applet (it's unsafe, but so is
that particular option of the applet, and mktemp() is the least unsafe
way of implementing that option), but forcing the linker to error out on
warnings breaks the build in this case.

Change-Id: I6e976e9b467d2241be4bb0f3628991d098c492f0