Add path to libjvm.dylib for Java 7 on OS X

Now that OpenJDK 7 is available on OS X, we can support
creating such a JVM via JNI, by linking to libjvm.dylib.
WIP: Remove mysterious OS-X-specific code

Otherwise, set_default_library_path never gets called
on OS X, leaving us no chance to link against OpenJDK 7.
Use create_java_vm code on OS X for non-Apple JVMs

Previously, the create_java_vm method excluded the bulk of the logic on
OS X, period, assuming that if we are running on OS X then we will find
and use an Apple JVM only.

However, on newer versions of OS X (particularly on OS X 10.9
Mavericks), all the Apple JVMs are removed by default, and the JavaVM
framework is no longer in place to support them. That is, in
/System/Library/Frameworks/JavaVM.Framework/Versions there is no longer
a '1.6' or '1.5' folder, and instead only 'A' with a 'Current' symlink
to 'A'. So all the logic in set_path_to_apple_JVM fails and the function
fails with a "Could not access Java VM" error.

Since we now return an error code for this situation, we can test for
it, and then continue to the non-Apple-specific logic in that case,
rather than always falling back to system Java.
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.