Ok, there's my problem, probably. The closest jdk I could find was 1.8.0._181, and even that gave me errors. I tried to use proguard 4.7 that came with Android Studio, unsuccessfully, and after that updated it to 7.1 to pretty much the same result. Well, the errors were different, but still it didn't build.
So, I'm trying to build the project, but the progress is slow. Apparently, it can be built only with specific matching versions of jdk, proguard and build tools. The question is: what versions of those should I use?