RFR: 8259942: Enable customizations in CompileJavaModules.gmk and Main.gmk

[Magnus Ihse Bursie]

On Mon, 18 Jan 2021 18:27:14 GMT, Adam Farley <afarley at openjdk.org> wrote:

> Ensure make files look on the include path or in PHASE_MAKEDIRS for
> customizations.
> 
> Change also adds a tidy-up that improves readability.

Marked as reviewed by ihse (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/2134