RFR: 8273072: Avoid using += in configure

[Jie Fu]

On Fri, 27 Aug 2021 10:21:26 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

> JDK-8272700 was created to fix a bug in a variable assignment in configure. While it fixed the bug, it kept the problematic syntax that caused the bug in the first place. 
> 
> We do not use the `FOO+="appended"` syntax for appending to variables in shell scripts, since this differs from what you'd expect (and what make produces) in that no space is added before the appended text. 
> 
> Instead, we use the longer, but clearer `FOO="$FOO appended"`.

Please update the copyright year in `jdk-version.m4`.

Shall we also fix this one?

./devkit/createMacosxDevkit.sh:    EXCLUDE_ARGS+="--exclude=$ex "

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PR: https://git.openjdk.java.net/jdk/pull/5276