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

[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

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

Changes: https://git.openjdk.java.net/jdk/pull/2982/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2982&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255790
  Stats: 96 lines in 2 files changed: 7 ins; 57 del; 32 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