[CMake] UseSWIG.cmake Parallel Build Issue

[Brian Panneton]

I have found an issue with UseSWIG.cmake during parallel builds. If you are
swigging Python and Java using the SWIG_ADD_MODULE and SWIG_LINK_LIBRARIES
macros you could run into this issue.

When you run through CMake, the macro SWIG_GET_EXTRA_OUTPUT_FILES sets the
variable SWIG_EXTRA_GENERATED_FILES. This variable is used by a custom
command as an output. When CMaking, it never gets reset. Thus if you start
with swigging Python, the file module.py is appended to it. Then when
swigging Java, the same file remains as an output.

If you are running in parallel, the output file is claimed by both Java and
Python. Somehow this makes it so the Python lib might not get fully built on
the first try.

To solve this issue on the user side we need to reset
SWIG_EXTRA_GENERATED_FILES right before you call the macro SWIG_ADD_MODULE,
like such:

set(swig_extra_generated_files "")
swig_add_module(${module_name} python module.i)

To solve this issue on the UseSWIG.cmake side add SET(${outfiles} "") to the
first line of the macro SWIG_GET_EXTRA_OUTPUT_FILES, like such:

MACRO(SWIG_GET_EXTRA_OUTPUT_FILES language outfiles generatedpath infile)
  SET(${outfiles} "")
  GET_SOURCE_FILE_PROPERTY(SWIG_GET_EXTRA_OUTPUT_FILES_module_basename
    ${infile} SWIG_MODULE_NAME)

I am pretty sure this would solve the issue. Please let me know if this is
the correct way to go about it.

Thanks,
Brian Panneton
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