gnu: gcc@4.9: Fix building with glibc@2.26.

* gnu/packages/gcc.scm (gcc@4.9)[source]: Add snippet to adjust
linux-unwind.h to changes in glibc.
* gnu/packages/commencement.scm (gcc-for-libstdc++): New variable.
(libstdc++-boot0): Inherit from gcc-for-libstdc++, update note.

gnu/packages/commencement.scm

@@ -3,7 +3,7 @@
 ;;; Copyright © 2014 Andreas Enge <andreas@enge.fr>
 ;;; Copyright © 2012 Nikita Karetnikov <nikita@karetnikov.org>
 ;;; Copyright © 2014, 2015, 2017 Mark H Weaver <mhw@netris.org>
-;;; Copyright © 2017 Efraim Flashner <efraim@flashner.co.il>
+;;; Copyright © 2017, 2018 Efraim Flashner <efraim@flashner.co.il>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -173,12 +173,28 @@
                     ,cf)))))
      (inputs %boot0-inputs))))
 
+;; gcc-4.9 was fixed late in the core-update cycle and so this GCC is only
+;; needed to prevent a full world rebuild, and can be replaced with gcc-4.9.
+(define gcc-for-libstdc++
+  (package (inherit gcc-4.9)
+    (version "4.9.4")
+    (source (origin
+              (method url-fetch)
+              (uri (string-append "mirror://gnu/gcc/gcc-"
+                                  version "/gcc-" version ".tar.bz2"))
+              (sha256
+               (base32
+                "14l06m7nvcvb0igkbip58x59w3nq6315k6jcz3wr9ch1rn9d44bc"))
+              (patches (search-patches "gcc-arm-bug-71399.patch"
+                                       "gcc-libvtv-runpath.patch"
+                                       "gcc-fix-texi2pod.patch"))))))
+
 (define libstdc++-boot0
   ;; GCC's libcc1 is always built as a shared library (the top-level
   ;; 'Makefile.def' forcefully adds --enable-shared) and thus needs to refer
   ;; to libstdc++.so.  We cannot build libstdc++-5.3 because it relies on
-  ;; C++14 features missing in our bootstrap compiler.
+  ;; C++14 features missing in some of our bootstrap compilers.
-  (let ((lib (package-with-bootstrap-guile (make-libstdc++ gcc-4.9))))
+  (let ((lib (package-with-bootstrap-guile (make-libstdc++ gcc-for-libstdc++))))
     (package
       (inherit lib)
       (name "libstdc++-boot0")

gnu/packages/gcc.scm

@@ -3,7 +3,7 @@
 ;;; Copyright © 2014, 2015 Mark H Weaver <mhw@netris.org>
 ;;; Copyright © 2014, 2015, 2016, 2017 Ricardo Wurmus <rekado@elephly.net>
 ;;; Copyright © 2015 Andreas Enge <andreas@enge.fr>
-;;; Copyright © 2015, 2016, 2017 Efraim Flashner <efraim@flashner.co.il>
+;;; Copyright © 2015, 2016, 2017, 2018 Efraim Flashner <efraim@flashner.co.il>
 ;;; Copyright © 2016 Carlos Sánchez de La Lama <csanchezdll@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
@@ -394,7 +394,18 @@ Go.  It also includes runtime support libraries for these languages.")
                 "14l06m7nvcvb0igkbip58x59w3nq6315k6jcz3wr9ch1rn9d44bc"))
               (patches (search-patches "gcc-arm-bug-71399.patch"
                                        "gcc-libvtv-runpath.patch"
-                                       "gcc-fix-texi2pod.patch"))))
+                                       "gcc-fix-texi2pod.patch"))
+              (modules '((guix build utils)))
+              ;; This is required for building with glibc-2.26.
+              ;; https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81712
+              (snippet
+               '(for-each
+                  (lambda (dir)
+                    (substitute* (string-append "libgcc/config/"
+                                                dir "/linux-unwind.h")
+                      (("struct ucontext") "ucontext_t")))
+                  '("aarch64" "alpha" "bfin" "i386" "m68k" "nios2"
+                    "pa" "sh" "tilepro" "xtensa")))))
     ;; Override inherited texinfo-5 with latest version.
     (native-inputs `(("perl" ,perl)   ;for manpages
                      ("texinfo" ,texinfo)))))