Fix "make install" for gnuplot: use --prefix.

Using configure-options here overrides cmmi's default, which is
"a --prefix option that points to the part directory".
(https://pypi.python.org/pypi/zc.recipe.cmmi).

Without the --prefix option, buildout will fail because it doesn't
have permission to install gnuplot system-wide.
Merge pull request #1 from khink/master

Fix gnuplot installation