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

[Brice Dutheil]

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.

Fair. I believe tool-chains support will improve in the future. IntelliJ for example don't explicitly support tool-chain but it will follow what is declared in Gradle, so this might be encoded as Gradle task which is indeed inefficient in some cases.
Maybe this can be revisited when tool-chains tooling is more solid and more compelling.

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