Fix ant/Java detection in cmake scripts

Mac OS X 10.7 and newer don't come with Java installed. They do
include some stub binaries, which ask the user if they want to
install Java when run.

OpenCV's cmake script just checks for the existence of an ant
binary and assumes that Java's available if ant is. As a result,
cmake will configure the build to use Java and it will fail once
it tries to compile the Java bindings.

This fixes the issue by checking for the exit status of
`ant -version` - it exits 0 if Java is installed, or 1
otherwise.
Merge pull request #694 from mistydemeo:fix_java_detection