Use platform version of libjpeg on Linux

libjavafx_iio.so includes a copy of libjpeg since that is not
available on Windows and perhaps not on Mac.
But on Linux we should be able to rely on it being installed as
part of the platform (JDK 11 does this).

So we can update the build so that on Linux
libjavafx_iio.so does not compile any of the JPEG source files
and instead links to the system libjpeg