gnu: glibc: Use 'modify-phases'.

* gnu/packages/base.scm (glibc)[arguments]: Use 'modify-phases' instead
  of 'alist-cons-before'.

gnu/packages/base.scm

@@ -512,7 +512,8 @@ store.")
             "libc_cv_ssp=no")
 
       #:tests? #f                                 ; XXX
-      #:phases (alist-cons-before
+      #:phases (modify-phases %standard-phases
+                 (add-before
                   'configure 'pre-configure
                   (lambda* (#:key inputs native-inputs outputs
                                   #:allow-other-keys)
@@ -560,8 +561,7 @@ store.")
                          ;; "bilingual" eval/exec magic at the top of the file.
                          "")
                         (("exec @PERL@")
-                       "exec perl"))))
-                %standard-phases)))
+                         "exec perl"))))))))
 
    (inputs `(("static-bash" ,static-bash)))