RFR: JDK-8257547: Handle multiple prereqs on the same line in deps files [v2]

[Erik Joelsson]

> After fixing JDK-8256810 and starting to look into backporting it, I discovered more potential failing cases. Certain versions of GCC may sometimes output multiple prerequisite files on the same line. I think the easiest way to handle this new issue is to split such lines.
> 
> When splitting lines, we need to make sure to not just split on any space. Some compilers output the : of the rule with a leading space, and already split lines will have a space before the backslash.

Erik Joelsson has updated the pull request incrementally with one additional commit since the last revision:

  Added test

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1548/files
  - new: https://git.openjdk.java.net/jdk/pull/1548/files/3164af58..9f8a7edb

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1548&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1548&range=00-01

  Stats: 71 lines in 2 files changed: 70 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1548.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1548/head:pull/1548

PR: https://git.openjdk.java.net/jdk/pull/1548