RFR: 8273072: Avoid using += in configure

[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"`.

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Commit messages:
 - 8273072: Avoid using += in configure

Changes: https://git.openjdk.java.net/jdk/pull/5276/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5276&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273072
  Stats: 5 lines in 3 files changed: 1 ins; 0 del; 4 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