Set libs property for tcl and tk (#9372)

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

@@ -93,6 +93,11 @@ def symlink_tclsh(self):
     # ========================================================================
     # Set up environment to make install easy for tcl extensions.
     # ========================================================================
+
+    @property
+    def libs(self):
+        return LibraryList([])
+
     @property
     def command(self):
         """Returns the tclsh command.

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

@@ -47,6 +47,10 @@ class Tk(AutotoolsPackage):
 
     configure_directory = 'unix'
 
+    @property
+    def libs(self):
+        return LibraryList([])
+
     def setup_environment(self, spack_env, run_env):
         # When using Tkinter from within spack provided python+tk, python
         # will not be able to find Tcl/Tk unless TK_LIBRARY is set.