Since commit v3.8.0-rc1~251^2 (UseSWIG: Record generated java files as
custom command outputs, 2016-11-28) the generated `.java` files are
listed as sources in the call to `add_library` by swig_add_library.
They are listed only as the outputs of custom commands and not intended
for compilation as part of the library.

Reported-by: Alan W. Irwin <irwin@beluga.phys.uvic.ca>