PROTON-550: Fail build if Ruby is enabled but missing dependencies.

Now, instead of just stating that a dependency is missing, CMake will
fail if it's trying to build Ruby but any dependency is missing.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1586400 13f79535-47bb-0310-9956-ffa450edef68