RFR: 8339335: set number of parallel jobs when building webkit [v2]

[Johan Vos]

> When building webkit, the perl script in `modules/javafx.web/src/main/native/Tools/Scripts/build-webkit` will check for a `-j` argument in `$makeArgs`. If that doesn't exist, it will default to `numberOfCPUs()`.
> This PR passes the `NUM_COMPILE_THREADS` from `build.gradle` to the `build-webkit` script. When not setting `NUM_COMPILE_THREADS` explicitly,it will default to the number of CPU's as well, hence it will have the same behavior as without this PR.
> When setting `NUM_COMPILE_THREADS` explicitly, the provided number will be used by cmake as well now.

Johan Vos has updated the pull request incrementally with one additional commit since the last revision:

  Remove the special case where native compile jobs are hard set to 1
  on Mac -- see JDK-8339505

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1554/files
  - new: https://git.openjdk.org/jfx/pull/1554/files/a6e6c1c9..1a1a360a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1554&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1554&range=00-01

  Stats: 7 lines in 1 file changed: 0 ins; 5 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/1554.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1554/head:pull/1554

PR: https://git.openjdk.org/jfx/pull/1554