Hi! I was able to fix the mingw issue through another way. The build matrix
is here https://ci.appveyor.com/project/amCap1712/pljava/builds/33715210/.
Appveyor does not have JDK 9 and 10. I have written a script for those
versions to install them. As you can see, Java 9, PG9 and PG10 are failing.
I am investigating these. Also, it seems that msys2 only has postgres12
pre-built packages. If we want to test for other versions, we may need to
build from source. What are your thoughts on this?