RFR: 7903136: Don't require jdk18_home to be set in jextract gradle build

[Maurizio Cimadamore]

On Mon, 28 Mar 2022 16:02:21 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

> Drop the requirement to set `jdk18_home` in the gradle build, and rely instead on `JAVA_HOME` being set to JDK 18.
> 
> I've also updated the build instructions. The `-Pjtreg_home` flag seems to have been dropped by accident from the test command. I've re-added it. Also renamed `libclang_dir` -> `llvm_dir` which was missed in an earlier change.

Not fully convinced about this. Java 18 works, yes. But what about 19 EA, or Panama EA (for foreign-preview changes) ? My feeling is that this project will always live a bit in the bleeding edge, so a more manual way to specify which JDK should be used should be possible, at the very least as an escape hatch.

Then, on a more subjective matter, I find setting JAVA_HOME and then calling the executable to be in the same league as specifying a custom property. But maybe there are some advantages - e.g.. maybe in terms of friendliness w.r.t. IDE support?

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PR: https://git.openjdk.java.net/jextract/pull/9