RFR: 8227808: Make GTK3 libraries mandatory for building on Linux

[Ambarish Rapte]

Need for the change [[JDK-8227808](https://bugs.openjdk.java.net/browse/JDK-8227808)]:
Currently GTK3 is the default on Linux. However, there is build logic that will skip building glassgtk3 if the gtk3 development libraries are not installed. This creates a situation where the build will succeed, but the resulting JavaFX library will fail to run unless you explicitly force GTK2 with "-Djdk.gtk.version=2". 

How to verify the change:
On a Linux system without gtk2 and gtk3 packages,
1. Clone jfx and run gradle sdk.
-> Build should fail with an error for missing GTK3 packages.

2. Install gtk3 packages, remove build directory and run gradle clean sdk.
-> Build should fail with an error for missing GTK2 packages.

3. Install gtk2 packages, remove build directory and run gradle clean sdk.
-> Build should finish without any error.

Build verification:
1. Run a JavaFX application with -Djdk.gtk.verbose=true
Verify the verbose message, the default glassgtk3 library should be used.

2. Run a JavaFX application with -Djdk.gtk.verbose=true -Djdk.gtk.version=2
Verify the verbose message, glassgtk2 library should be used.

3. Run a JavaFX application with -Djdk.gtk.verbose=true -Djdk.gtk.version=3
Verify the verbose message, glassgtk3 library should be used.

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Commits:
 - 57157900: 8227808: Make GTK3 libraries mandatory for building on Linux

Changes: https://git.openjdk.java.net/jfx/pull/61/files
 Webrev: https://webrevs.openjdk.java.net/jfx/61/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8227808
  Stats: 18 lines in 1 file changed: 0 ins; 10 del; 8 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