Bootstrap environment-modules

Renames the existing bootstrap command to 'clone'. Repurposes
'spack bootstrap' to install packages that are useful to the
operation of Spack (for now this is just environment-modules).
For bash and ksh users running setup-env.sh, if a Spack-installed
instance of environment-modules is detected and environment modules
and dotkit are not externally available, Spack will define the 
'module' command in the user's shell to use the environment-modules
built by Spack.

var/spack/repos/builtin/packages/environment-modules/package.py

@@ -35,6 +35,8 @@ class EnvironmentModules(Package):
 
     version('3.2.10', '8b097fdcb90c514d7540bb55a3cb90fb')
 
+    variant('X', default=True, description='Build with X functionality')
+
     # Dependencies:
     depends_on('tcl', type=('build', 'link', 'run'))
 
@@ -75,6 +77,9 @@ def install(self, spec, prefix):
             'CPPFLAGS=' + ' '.join(cpp_flags)
         ]
 
+        if '~X' in spec:
+            config_args = ['--without-x'] + config_args
+
         configure(*config_args)
         make()
         make('install')