[11u] RFR 8257547: Handle multiple prereqs on the same line in deps files

[Roland Westrelin]

Hi,
 
Original bug:
 https://bugs.openjdk.java.net/browse/JDK-8257547
 https://github.com/openjdk/jdk/commit/36209b70
 
Original patch does not apply cleanly to 11u because in 16:
- solaris support was dropped
- this patch was on top of JDK-8256810 which it reworks entirely (so
backporting it first doesn't seem to be worth it)
 
11u webrev:
  http://cr.openjdk.java.net/~roland/8257547.11u/

I cherry-picked the comment from JDK-8256810.
 
Testing: x86_64 build, included test
 
Thanks,
-Monty