petsc: explicitly disable X11 (#2533)

otherwise petsc may pickup libX11 on login nodes during config/build,
but libX11 may not be available on computation nodes.

var/spack/repos/builtin/packages/petsc/package.py

@@ -130,6 +130,7 @@ def mpi_dependent_options(self):
 
     def install(self, spec, prefix):
         options = ['--with-ssl=0',
+                   '--with-x=0',
                    '--download-c2html=0',
                    '--download-sowing=0',
                    '--download-hwloc=0']