The CTEST_RUN_Java option added by commit c8f39193 (Avoid problem
reading jni.h on Macs, 2010-10-25) was a quick hack to disable the Java
test on Mac machines after an update from Apple created a broken jni.h
symlink.  Remove the option and instead test whether jni.h exists as a
readable file before reading it.  This restores the original Java test
enabling logic but makes it robust to the broken symlink.