RFR: 8273072: Avoid using += in configure [v2]

[Magnus Ihse Bursie]

> 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"`.

Magnus Ihse Bursie has updated the pull request incrementally with one additional commit since the last revision:

  Fix code review comments

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5276/files
  - new: https://git.openjdk.java.net/jdk/pull/5276/files/a8e0c35a..3083307d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5276&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5276&range=00-01

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5276.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5276/head:pull/5276

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