Port the detection of zlib from the main autoconf system to the sample
project's autoconf. This is the last of the missing optional checks used
by libSupport that seemed to be missing from the sample project, but
I could easily have missed some as this was done by inspection when
Craig asked me to add the terminfo support.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188618 91177308-0d34-0410-b5e6-96231b3b80d8