When an interpreter is created, [incr Tk] initializes this
variable to hold the name of a directory containing the system library
of [incr Tk] scripts. The initial value of itk::library
is set from the ITK_LIBRARY environment variable if it exists,
or from a compiled-in value otherwise.
When [incr Tk] is added to an interpreter, it executes
the script "init.itk" in this directory. This script,
in turn, looks for other script files with the name "init.xxx".
Mega-widget libraries will be automatically initialized if they
install a script named "init.xxx" in this directory,
where "xxx" is the name of the mega-widget library.
For example, the [incr Widgets] library installs
the script "init.iwidgets" in this directory.
This script establishes the "iwidgets" namespace, and sets
up autoloading for all [incr Widgets] commands.