gnu: tcl: Use 'modify-phases' syntax.

* gnu/packages/tcl.scm (tcl)[arguments]: Use 'modify-phases'.

gnu/packages/tcl.scm

@@ -48,12 +48,10 @@
                 "19bb09l55alz4jb38961ikd5116q80s51bjvzqy44ckkwf28ysvw"))))
     (build-system gnu-build-system)
     (arguments
-     '(#:phases (alist-cons-before
-                 'configure 'pre-configure
-                 (lambda _
-                   (chdir "unix"))
-                 (alist-cons-after
-                  'install 'install-private-headers
+     '(#:phases (modify-phases %standard-phases
+                  (add-before 'configure 'pre-configure
+                    (lambda _ (chdir "unix") #t))
+                 (add-after 'install 'install-private-headers
                    (lambda* (#:key outputs #:allow-other-keys)
                      ;; Private headers are needed by Expect.
                      (and (zero? (system* "make"
@@ -64,8 +62,8 @@
                             ;; Programs such as Ghostscript rely on it.
                             (with-directory-excursion bin
                               (symlink (car (find-files "." "tclsh"))
-                                      "tclsh")))))
-                  %standard-phases))
+                                       "tclsh"))
+                            #t)))))
 
        ;; By default, man pages are put in PREFIX/man, but we want them in
        ;; PREFIX/share/man.  The 'validate-documentation-location' phase is