Add variants for tk package (spack#11956)

TK can be built with support for libXft and X Screensaver. Both of these
are turned on in the Tk configure script. That means the system
libraries will get picked up if they are on the system and nothing is
specified in the package. Since the default for both of them is 'True' I
set the default value for the variants to 'True'.

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

@@ -23,10 +23,17 @@ class Tk(AutotoolsPackage):
     version('8.6.3', '85ca4dbf4dcc19777fd456f6ee5d0221')
     version('8.5.19', 'e89df710447cce0fc0bde65667c12f85')
 
+    variant('xft', default=True,
+            description='Enable X FreeType')
+    variant('xss', default=True,
+            description='Enable X Screen Saver')
+
     extends('tcl')
 
     depends_on('tcl@8.6:', when='@8.6:')
     depends_on('libx11')
+    depends_on('libxft', when='+xft')
+    depends_on('libxscrnsaver', when='+xss')
 
     configure_directory = 'unix'
 
@@ -65,9 +72,15 @@ def setup_dependent_environment(self, spack_env, run_env, dependent_spec):
 
     def configure_args(self):
         spec = self.spec
-        return ['--with-tcl={0}'.format(spec['tcl'].prefix.lib),
-                '--x-includes={0}'.format(spec['libx11'].prefix.include),
-                '--x-libraries={0}'.format(spec['libx11'].prefix.lib)]
+        config_args = [
+            '--with-tcl={0}'.format(spec['tcl'].prefix.lib),
+            '--x-includes={0}'.format(spec['libx11'].prefix.include),
+            '--x-libraries={0}'.format(spec['libx11'].prefix.lib)
+        ]
+        config_args += self.enable_or_disable('xft')
+        config_args += self.enable_or_disable('xss')
+
+        return config_args
 
     @run_after('install')
     def symlink_wish(self):