RFR: 8255790: GTKL&F: Java 16 crashes on initialising GTKL&F on Manjaro Linux [v3]

[Phil Race]

> From a build perspective this partially reverts https://bugs.openjdk.java.net/browse/JDK-8249821 except that it keeps 
> the harfbuzz sources separate and still supports building and running against a system harfbuzz which is only of interest or relevance on Linux.
> 
> I ended up having to go this way because its is the least unsatisfactory solution.
> I did not want us to build a devkit to link against a system linux only to find we couldn't use it at runtime
> because too many systems have to old a version of harfbuzz.
> 
> This solves the Manjaro Linux problem and I've manually verified building against a system hardbuxz on Ubuntu 20.10
> 
> There are couple of incidental fixes in here too
> - "libharfbuzz" should not have been in the EXTRA_HEADERS var when building against a system version
> - harfbuzz/hb-ucdn is gone and should not be listed as a header directory needed to build the bundled copy
> - I expect it also resolves https://bugs.openjdk.java.net/browse/JDK-8262502

Phil Race has updated the pull request incrementally with one additional commit since the last revision:

  8255790: GTKL&F: Java 16 crashes on initialising GTKL&F on Manjaro Linux

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2982/files
  - new: https://git.openjdk.java.net/jdk/pull/2982/files/f668b327..a92a146c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2982&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2982&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2982.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2982/head:pull/2982

PR: https://git.openjdk.java.net/jdk/pull/2982