[Rev 01] RFR: 8227808: Make GTK3 libraries mandatory for building on Linux

[Ambarish Rapte]

The pull request has been updated with additional changes.

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Added commits:
 - 3a78eb04: remove gtk flags from launcher ccflags

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/61/files
  - new: https://git.openjdk.java.net/jfx/pull/61/files/57157900..3a78eb04

Webrevs:
 - full: https://webrevs.openjdk.java.net/jfx/61/webrev.01
 - incr: https://webrevs.openjdk.java.net/jfx/61/webrev.00-01

  Issue: https://bugs.openjdk.java.net/browse/JDK-8227808
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/61.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/61/head:pull/61

PR: https://git.openjdk.java.net/jfx/pull/61