gnu: gcc: Allow cross-compilation of GCC itself.

* gnu/packages/gcc.scm (gcc-4.7): Define `CC_FOR_TARGET',
  `AR_FOR_TARGET', etc. when cross-compiling.

gnu/packages/gcc.scm

@@ -32,7 +32,39 @@
   "ftp://gcc.gnu.org/pub/gcc/infrastructure/")
 
 (define-public gcc-4.7
-  (let ((stripped? #t))                         ; TODO: make this a parameter
+  (let* ((stripped? #t)                           ; TODO: make this a parameter
+         (maybe-target-tools
+          (lambda ()
+            ;; Return the `_FOR_TARGET' variables that are needed when
+            ;; cross-compiling GCC.
+            (let ((target (%current-target-system)))
+              (if target
+                  (map (lambda (var tool)
+                         (string-append (string-append var "_FOR_TARGET")
+                                        "=" target "-" tool))
+                       '("CC"  "CXX" "LD" "AR" "NM" "RANLIB" "STRIP")
+                       '("gcc" "g++" "ld" "ar" "nm" "ranlib" "strip"))
+                  '()))))
+         (configure-flags
+          (lambda ()
+            ;; This is terrible.  Since we have two levels of quasiquotation,
+            ;; we have to do this convoluted thing just so we can insert the
+            ;; contents of (maybe-target-tools).
+            (list 'quasiquote
+                  (append
+                   '("--enable-plugin"
+                     "--enable-languages=c,c++"
+                     "--disable-multilib"
+
+                     "--with-local-prefix=/no-gcc-local-prefix"
+
+                     ,(let ((libc (assoc-ref %build-inputs "libc")))
+                        (if libc
+                            (string-append "--with-native-system-header-dir=" libc
+                                           "/include")
+                            "--without-headers")))
+
+                   (maybe-target-tools))))))
     (package
       (name "gcc")
       (version "4.7.3")
@@ -54,18 +86,7 @@
       (arguments
        `(#:out-of-source? #t
          #:strip-binaries? ,stripped?
-        #:configure-flags
-        `("--enable-plugin"
-          "--enable-languages=c,c++"
-          "--disable-multilib"
-
-          "--with-local-prefix=/no-gcc-local-prefix"
-
-          ,(let ((libc (assoc-ref %build-inputs "libc")))
-             (if libc
-                 (string-append "--with-native-system-header-dir=" libc
-                                "/include")
-                 "--without-headers")))
+         #:configure-flags ,(configure-flags)
          #:make-flags
          (let ((libc (assoc-ref %build-inputs "libc")))
            `(,@(if libc