Placeholder for simplified system library exception.
The definition of "System Libraries" in GPLv3 (used in the
Corresponding Source definition) is unnecessarily complex and
difficult to parse and understand. This commit replaces it with a
definition based on the GPLv2 system library exception but with a few
changes adapted from the version of the system library exception in
the first dicussion draft of GPLv3.
Further work on this definition is necessary; this version can be seen
as something of a placeholder.