[foreign] RFR 8215570: jextract should warn when no output is emitted

[Maurizio Cimadamore]

Hi,
as a result of the recentr refactoring, it is now possible that jextract 
will detect that no declarations will be added to the jextract output. 
In the past we used to generate an empty header, but, because of how the 
new pipeline works, we actually end up with nothing now.

I think the new behavior is acceptable but there are two issues with it:

1) no warning is emitted
2) jextract will still put jextract.properties in the output jar/folder

This patch fix both problems:

http://cr.openjdk.java.net/~mcimadamore/panama/8215570_v2/

Maurizio