RFR: 7903136: Don't require jdk18_home to be set in jextract gradle build [v4]

[Athijegannathan Sundararajan]

On Tue, 29 Mar 2022 18:29:55 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.
>
> Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Tabs to spaces

README.md line 10:

> 8: 
> 9: The JDK used to build jextract can be specified with `-Pjdk=/path/to/jdk`. If that option is omitted, a build JDK will be downloaded automatically.
> 10: 

Should we say JDK used to build jextract should be jdk 18 ?

build.gradle line 93:

> 91:     println("using JDK from 'jdk' project property");
> 92:     jdkHome = project.property("jdk");
> 93:     checkJDKRelease(jdkHome, minimalJDKVersion);

we check minimum JDK. So.. jdk 19 will be accepted even after foreign-preview hits jdk 19. But that won't work with this version of jdk. I think it is better to accept only jdk 18 - as this jextract code/test/samples work only with jdk 18.

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