RFR: 8276746: Add section on reproducible builds in building.md [v2]

[Magnus Ihse Bursie]

> Reproducible builds are all the vogue. The JDK has been making great strides in getting there, but still has some way to go. However, to get as close as possible, some special configuration is needed. 
> 
> This has been "tribal knowledge" of a few persons in the build team. It needs to be properly documented to help other users wanting to create reproducible builds of the JDK.

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

  Use shell syntax, not make

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6279/files
  - new: https://git.openjdk.java.net/jdk/pull/6279/files/cfb154b3..d8698421

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6279&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6279&range=00-01

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6279.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6279/head:pull/6279

PR: https://git.openjdk.java.net/jdk/pull/6279