Warn when a non-reproducible compiler is used

[io7m]

Basically, warn when the user isn't using OpenJDK 11 (or whatever is the current LTS release). The reason: Compilers now insert platform module versions into compiled source files, so the output is not reproducible in the sense that the user will get the version numbers of their own platform's modules inserted into the code.

A warning such as:

[WARN] Compiler is not OpenJDK 11.0: Produced bytecode may not be byte-for-byte identical to the published bytecode. See https://github.com/io7m/primogenitor#reproducibility

... will serve as documentation to those trying to reproduce code that is identical to the published code.