Colin Watson reports that linking with 'gold' doesn't work, because

PuTTY makes explicit use of libX11 without including -lX11 on the
link line. (GNU ld appears to pull in libX11 automatically because
it's needed for the dependencies of GTK, but gold expects that
dependency to be satisfied at run time via DT_NEEDED and hence
doesn't bother.) Hence, add explicit -lX11 to both Makefile.gtk and
the autoconf world.

git-svn-id: svn://svn.tartarus.org/sgt/putty@8876 cda61777-01e9-0310-a592-d414129be87e