UseSWIG: Add option to specify swig output file directory

`swig` has two output-related options:

* `-o <outfile>`: Set name of C/C++ output file to <outfile>
* `-outdir <dir>`: Set language-specific files output directory to <dir>

We already have `CMAKE_SWIG_OUTDIR` for the latter.  Add a new
`SWIG_OUTFILE_DIR` option for the former.