Apple released an update recently in which there
was a broken symlink for jni.h that pointed to
a non-existent file. CMake had trouble configuring
because it encountered an error trying to read
jni.h.

We avoid the problem here by introducing a variable
CTEST_RUN_Java, which defaults to OFF on Macs, that
indicates whether to even attempt finding java, reading
jni.h and adding the "Java" CMake test.

If you would like to test Java on a Mac where the
jni.h symlink is not busted, feel free to pass in
-DCTEST_RUN_Java:BOOL=ON when configuring CMake.