utils: Add 'cxx-for-target'.

* guix/utils.scm (cxx-for-target): New procedure.

guix/utils.scm

@@ -78,6 +78,7 @@
             target-arm?
             target-64bit?
             cc-for-target
+            cxx-for-target
 
             version-compare
             version>?
@@ -542,6 +543,11 @@ a character other than '@'."
       (string-append target "-gcc")
       "gcc"))
 
+(define* (cxx-for-target #:optional (target (%current-target-system)))
+  (if target
+      (string-append target "-g++")
+      "g++"))
+
 (define version-compare
   (let ((strverscmp
          (let ((sym (or (dynamic-func "strverscmp" (dynamic-link))