gnu: gcc-static: Remove -lgcc_s from GNU_USER_TARGET_LIB_SPEC.

* gnu/packages/make-bootstrap.scm (%gcc-static): Add 'remove-lgcc_s
  phase.
* gnu/packages/gcc.scm (gcc-4.7): Add comment.

gnu/packages/gcc.scm

 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2012, 2013, 2014 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2014, 2015 Mark H Weaver <mhw@netris.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -207,6 +208,11 @@ (define-public gcc-4.7
                    ;; libgcc_s.so when pthread_cancel support is needed, but
                    ;; having it in the application's RUNPATH isn't enough; see
                    ;; <http://sourceware.org/ml/libc-help/2013-11/msg00023.html>.)
+                   ;;
+                   ;; NOTE: The '-lgcc_s' added below needs to be removed in a
+                   ;; later phase of %gcc-static.  If you change the string
+                   ;; below, make sure to update the relevant code in
+                   ;; %gcc-static package as needed.
                    (format #f "#define GNU_USER_TARGET_LIB_SPEC \
 \"-L~a/lib %{!static:-rpath=~a/lib %{!static-libgcc:-rpath=~a/lib64 -rpath=~a/lib -lgcc_s}} \" ~a"
                            libc libc libdir libdir suffix))

gnu/packages/make-bootstrap.scm

@@ -430,7 +430,19 @@ (define %gcc-static
                        "--disable-libquadmath"
                        "--disable-decimal-float")
                       (remove (cut string-match "--(.*plugin|enable-languages)" <>)
-                              ,flags))))))
+                              ,flags)))
+            ((#:phases phases)
+             `(alist-cons-after
+               'pre-configure 'remove-lgcc_s
+               (lambda _
+                 ;; Remove the '-lgcc_s' added to GNU_USER_TARGET_LIB_SPEC in
+                 ;; the 'pre-configure phase of our main gcc package, because
+                 ;; that shared library is not present in this static gcc.  See
+                 ;; <https://lists.gnu.org/archive/html/guix-devel/2015-01/msg00008.html>.
+                 (substitute* (find-files "gcc/config"
+                                          "^gnu-user.*\\.h$")
+                   ((" -lgcc_s}}") "}}")))
+               ,phases)))))
      (native-inputs
       (if (%current-target-system)
           `(;; When doing a Canadian cross, we need GMP/MPFR/MPC both