if that file exists, the NativeBigInteger will act as if jbigi.impl was set to the contents of that file.
For instance, a jbigi.cfg containing "win-p4" would have the NativeBigInteger search the classpath for the "win-p4" file and use it as a native library.
The jbigi.ref preempts the jbigi.impl property (only if the file exists and is not empty), but the external platform specific jbigi
preempts this (e.g. jbigi.dll or libjbigi.so), as does the jbigi.enable flag.
This option lets us have the admin console write to a file to choose which jbigi to use, rather than have to parse some shell script, etc
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