Allow compiling without GTK 3.0 being installed

After JDK-8163496 on Linux GTK 3.0 must be installed despite there being conditionals in the scripts to compile if it is not. There is a simple fix:

diff -r d59805c7ee03 buildSrc/linux.gradle
--- a/buildSrc/linux.gradle Tue Jan 03 10:39:05 2017 -0800
+++ b/buildSrc/linux.gradle Tue Oct 20 09:23:23 2015 -0700
@@ -181,7 +181,7 @@
     logger.info("Building libglassgtk3")
     LINUX.glass.variants += "glassgtk3"
 } else {
- logger.warning("NOT Building libglassgtk3")
+ logger.info("NOT Building libglassgtk3")
 }
 
 FileTree ft_gtk_launcher = fileTree("${project(":graphics").projectDir}/src/main/native-glass/gtk/") {