RFR: Improve and modernize the makefile [v2]

Tue Mar 16 16:14:13 UTC 2021

On Tue, 16 Mar 2021 13:30:05 GMT, Jesper Wilhelmsson <jwilhelm at openjdk.org> wrote:

>> Magnus Ihse Bursie has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Additional improvements for reusability and debuggability
>
> Looks good!

Yay! :-D The whitespace check worked!

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