RFR: Improve and modernize the makefile [v2]

[Magnus Ihse Bursie]

On Tue, 16 Mar 2021 16:11:26 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

>> Looks good!
>
> Yay! :-D The whitespace check worked!

Actually, it did not. There was nothing wrong the makefile, but the jcheck was incorrect. :( See https://github.com/openjdk/guide/pull/51

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PR: https://git.openjdk.java.net/guide/pull/46