RFR: 8246357: Allow static build of webkit library on linux

[Johan Vos]

add changes to build a static version of libjfxwebkit.a
Fox for JDK-8246357

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Commit messages:
 - add changes to build a static version of libjfxwebkit.a

Changes: https://git.openjdk.java.net/jfx/pull/245/files
 Webrev: https://webrevs.openjdk.java.net/jfx/245/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8246357
  Stats: 20 lines in 4 files changed: 17 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jfx/pull/245.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/245/head:pull/245

PR: https://git.openjdk.java.net/jfx/pull/245