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.

As an alternative to property or JAVA_HOME, did you consider using [Gradle java toolchains](https://docs.gradle.org/current/userguide/toolchains.html) ?

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