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

[Maurizio Cimadamore]

On Tue, 29 Mar 2022 09:08:54 GMT, Brice Dutheil <duke at openjdk.java.net> wrote:

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

This was proposed here:
https://mail.openjdk.java.net/pipermail/panama-dev/2022-March/016665.html

While for regular projects toolchains is great - I'm not sure it seems to limiting here. If I specify `JavaLanguageVersion.of(19)` I get an error:

> Unable to download toolchain matching these requirements: {languageVersion=19, vendor=any, implementation=vendor-specific}

Which is understandable, given 19 doesn't really exists yet. I think what we'd like would be some kind of toolchain-like solution, but which is backed by https://jdk.java.net/

Otherwise, we're back to finding workarounds to override what toolchains does, and manually set a JDK (at least in some cases).

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