Update to new upstream release 0.68 (security fixes)

Additional minor updates on top of cy@'s patch (USE_GNOME, USE_XORG,
LDFLAGS).

ChangeLog: http://www.chiark.greenend.org.uk/~sgtatham/putty/changes.html

Submitted by: cy@
MFH: 2017Q1
Security: CVE-2017-6542
Security: 9b973e97-0a99-11e7-ace7-080027ef73ec
Differential Revision: https://reviews.freebsd.org/D10023