gnu: Add libstdc++ as a standalone package.

* gnu/packages/gcc.scm (make-libstdc++): New procedure.
  (libstdc++-4.9): New variable.
* gnu/packages/commencement.scm (libstdc++): Add TODO comment.

gnu/packages/commencement.scm

@@ -497,6 +497,7 @@ (define binutils-final
 (define libstdc++
   ;; Intermediate libstdc++ that will allow us to build the final GCC
   ;; (remember that GCC-BOOT0 cannot build libstdc++.)
+  ;; TODO: Write in terms of 'make-libstdc++'.
   (package-with-bootstrap-guile
    (package (inherit gcc-4.8)
      (name "libstdc++")

gnu/packages/gcc.scm

@@ -347,6 +347,32 @@ (define-public gcc-5.1
                             '("gcc-arm-link-spec-fix.patch"
                               "gcc-5.0-libvtv-runpath.patch")))))))
 
+(define-public (make-libstdc++ gcc)
+  "Return a libstdc++ package based on GCC.  The primary use case is when
+using compilers other than GCC."
+  (package
+    (inherit gcc)
+    (name "libstdc++")
+    (arguments
+     `(#:out-of-source? #t
+       #:phases (alist-cons-before
+                 'configure 'chdir
+                 (lambda _
+                   (chdir "libstdc++-v3"))
+                 %standard-phases)
+       #:configure-flags `("--disable-libstdcxx-pch"
+                           ,(string-append "--with-gxx-include-dir="
+                                           (assoc-ref %outputs "out")
+                                           "/include"))))
+    (outputs '("out" "debug"))
+    (inputs '())
+    (native-inputs '())
+    (propagated-inputs '())
+    (synopsis "GNU C++ standard library")))
+
+(define-public libstdc++-4.9
+  (make-libstdc++ gcc-4.9))
+
 (define* (custom-gcc gcc name languages #:key (separate-lib-output? #t))
   "Return a custom version of GCC that supports LANGUAGES."
   (package (inherit gcc)