Respect JAVA_HOME on OS X
Respect JAVA_HOME on OS X

If the call to set_path_to_apple_JVM succeeds (i.e., returns 0), that
means we found an Apple JVM. But when JAVA_HOME is set, we do not *want*
that Apple Framework JVM to be used, but rather would prefer to use the
JVM indicated by JAVA_HOME.

So let's make set_path_to_apple_JVM return a failure code if JAVA_HOME
is set, so that we can proceed with the non-Apple-specific JVM logic.

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