FindJava: Update check to avoid accepting macOS stub 'java' as Java

Since commit dd378258 (FindJava: Do not accept OS X stub 'java' as
Java, 2014-10-24, v3.1.0-rc3~29^2) we try to avoid using the macOS
`/usr/bin/java` stub if no underlying implementation of Java is actually
installed.  However, the message that `/usr/bin/java` prints when there
is no Java available has changed since then.  Update our check to also
look for the new message.

While at it, revise the way we suppress `Java_JAVA_EXECUTABLE`.
Previously we set its cache entry to `Java_JAVA_EXECUTABLE-NOTFOUND`,
but that would cause the same find-and-reject sequence to be followed
every time CMake runs in a build tree.  Instead, use the approach from
commit 2c0db404 (FindSubversion: Do not accept macOS stub without
Xcode implementation, 2020-05-28, v3.18.0-rc1~67^2).  Leave the cache
entry alone and just set a normal variable of the same name to hide it.