Tell R's configure about tcltk config (#7085)

* Tell R's configure about tcltk config

Add configure arguments that specify the location of
the tcl and tk config scripts.

Fixes #7072

* Flake8 cleanup

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

@@ -96,11 +96,16 @@ def configure_args(self):
         spec   = self.spec
         prefix = self.prefix
 
+        tclConfig_path = join_path(spec['tcl'].prefix.lib, 'tclConfig.sh')
+        tkConfig_path = join_path(spec['tk'].prefix.lib, 'tkConfig.sh')
+
         config_args = [
             '--libdir={0}'.format(join_path(prefix, 'rlib')),
             '--enable-R-shlib',
             '--enable-BLAS-shlib',
-            '--enable-R-framework=no'
+            '--enable-R-framework=no',
+            '--with-tcl-config={0}'.format(tclConfig_path),
+            '--with-tk-config={0}'.format(tkConfig_path),
         ]
 
         if '+external-lapack' in spec: