RFR: JDK-8256810: Incremental rebuild broken on Macosx

[Erik Joelsson]

After fixing JDK-8256751, I noticed that the fix-deps-file macro isn't actually doing the right thing at all. It seems clang on Macosx is inconsistent with outputting relative or absolute paths in the deps files, so some files end up with double WORKSPACE_ROOT in the paths. It's also not handling the trailing slash after WORKSPACE_ROOT correctly, so some files are missing the slash between WORKSPACE_ROOT and the original relative path.

This time I have checked the output carefully.

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Commit messages:
 - Rewrite fix-deps-file macro

Changes: https://git.openjdk.java.net/jdk/pull/1360/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1360&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256810
  Stats: 8 lines in 1 file changed: 5 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1360.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1360/head:pull/1360

PR: https://git.openjdk.java.net/jdk/pull/1360