Opening a shared object in global mode risks conflicts with
other shared objects, but opening only one library that way
will hopefully not create conflicts.
(cherry picked from commit b29a7ae399)
The main advantage of GTK+ 3 is better support for HiDPI
displays. If GTK+ 3 libraries are not available or if the
`PLT_GTK2` environment variable is defined, GTK+ 2 is used
as before.