FXPackager: Improve Linux's JRE detecion

[ST-DDT]

The issue

When trying to launch natively packaged applications without bundled JRE. I get the following exception:

SomeProject Failed to locate JNI_CreateJavaVM
SomeProject Failed to launch JVM

This error is caused by the JRE_HOME environment variable not being set. Unfortunately the JRE_HOME won't be set by any installer on linux (or windows). apt-get install java-8-installer (At least not on any of my computers)

The solution

Search for system JRE's also:

• on JAVA_HOME only if JRE_HOME is not set
• on default locations even if JRE_HOME is not set

  References

• https://github.com/javafx-maven-plugin/javafx-maven-plugin/issues/222
• https://github.com/teamfx/openjfx-9-dev-rt/pull/3
• http://bugs.java.com/bugdatabase/view_bug.do?bug_id=JDK-8163356

[mkarg]

This is only a read-only mirror, so pull requests cannot be processed. Instead please describe your proposal as an issue. Thanks.