gnu: Default to GCC 5.

This reinstates and adjusts
commit e3d0fcbf7e.

* gnu/packages/patches/gcc-libiberty-printf-decl.patch: New file.
* gnu/packages/gcc.scm (gcc-5)[source]: Use it.
[patches]: Add "gcc-arm-bug-71399.patch".
(gcc): Switch to GCC-5.
* gnu/packages/commencement.scm (libstdc++-boot0): New variable.
(gcc-boot0)[inputs]: Add it.
(gcc-toolchain-4.9): Switch to GCC-4.9.
(gcc-toolchain-5): Switch to GCC-FINAL.
* gnu/local.mk (dist_patch_DATA): Add 'gcc-libiberty-printf-decl.patch'.

gnu/local.mk

@@ -546,6 +546,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/gcc-arm-bug-71399.patch			\
   %D%/packages/patches/gcc-arm-link-spec-fix.patch		\
   %D%/packages/patches/gcc-cross-environment-variables.patch	\
+  %D%/packages/patches/gcc-libiberty-printf-decl.patch		\
   %D%/packages/patches/gcc-libvtv-runpath.patch			\
   %D%/packages/patches/gcc-strmov-store-file-names.patch	\
   %D%/packages/patches/gcc-5.0-libvtv-runpath.patch		\

gnu/packages/commencement.scm

@@ -172,6 +172,26 @@
                     ,cf)))))
      (inputs %boot0-inputs))))
 
+(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.
+  (let ((lib (package-with-bootstrap-guile (make-libstdc++ gcc-4.9))))
+    (package
+      (inherit lib)
+      (name "libstdc++-boot0")
+      (arguments
+       `(#:guile ,%bootstrap-guile
+         #:implicit-inputs? #f
+
+         ;; XXX: libstdc++.so NEEDs ld.so for some reason.
+         #:validate-runpath? #f
+
+         ,@(package-arguments lib)))
+      (inputs %boot0-inputs)
+      (native-inputs '()))))
+
 (define gcc-boot0
   (package-with-bootstrap-guile
    (package (inherit gcc)
@@ -257,6 +277,9 @@
                ("mpc-source" ,(package-source mpc))
                ("binutils-cross" ,binutils-boot0)
 
+               ;; The libstdc++ that libcc1 links against.
+               ("libstdc++" ,libstdc++-boot0)
+
                ;; Call it differently so that the builder can check whether
                ;; the "libc" input is #f.
                ("libc-native" ,@(assoc-ref %boot0-inputs "libc"))
@@ -986,10 +1009,10 @@ and binaries, plus debugging symbols in the 'debug' output), and Binutils.")
   (gcc-toolchain gcc-4.8))
 
 (define-public gcc-toolchain-4.9
-  (gcc-toolchain gcc-final))
+  (gcc-toolchain gcc-4.9))
 
 (define-public gcc-toolchain-5
-  (gcc-toolchain gcc-5))
+  (gcc-toolchain gcc-final))
 
 (define-public gcc-toolchain-6
   (gcc-toolchain gcc-6))

gnu/packages/gcc.scm

@@ -358,7 +358,8 @@ Go.  It also includes runtime support libraries for these languages.")
               (sha256
                (base32
                 "0fihlcy5hnksdxk0sn6bvgnyq8gfrgs8m794b1jxwd1dxinzg3b0"))
-              (patches (search-patches "gcc-strmov-store-file-names.patch"
+              (patches (search-patches "gcc-arm-bug-71399.patch"
+                                       "gcc-strmov-store-file-names.patch"
                                        "gcc-5.0-libvtv-runpath.patch"))))))
 
 (define-public gcc-6
@@ -377,7 +378,7 @@ Go.  It also includes runtime support libraries for these languages.")
 
 ;; Note: When changing the default gcc version, update
 ;;       the gcc-toolchain-* definitions accordingly.
-(define-public gcc gcc-4.9)
+(define-public gcc gcc-5)
 
 (define-public (make-libstdc++ gcc)
   "Return a libstdc++ package based on GCC.  The primary use case is when

gnu/packages/patches/gcc-libiberty-printf-decl.patch

@@ -0,0 +1,28 @@
+This patch makes the exeception specifier of libiberty's 'asprintf'
+and 'vasprintf' declarations match those of glibc to work around the
+problem described at <https://gcc.gnu.org/ml/gcc-help/2016-04/msg00039.html>.
+
+The problem in part stems from the fact that libiberty is configured
+without _GNU_SOURCE (thus, it sets HAVE_DECL_ASPRINTF to 0), whereas libcc1
+is configured and built with _GNU_SOURCE, hence the conflicting declarations.
+
+--- gcc-5.3.0/include/libiberty.h	2016-04-23 22:45:46.262709079 +0200
++++ gcc-5.3.0/include/libiberty.h	2016-04-23 22:45:37.110635439 +0200
+@@ -625,7 +625,7 @@ extern int pwait (int, int *, int);
+ /* Like sprintf but provides a pointer to malloc'd storage, which must
+    be freed by the caller.  */
+ 
+-extern int asprintf (char **, const char *, ...) ATTRIBUTE_PRINTF_2;
++extern int asprintf (char **, const char *, ...) __THROWNL ATTRIBUTE_PRINTF_2;
+ #endif
+ 
+ /* Like asprintf but allocates memory without fail. This works like
+@@ -637,7 +637,7 @@ extern char *xasprintf (const char *, ..
+ /* Like vsprintf but provides a pointer to malloc'd storage, which
+    must be freed by the caller.  */
+ 
+-extern int vasprintf (char **, const char *, va_list) ATTRIBUTE_PRINTF(2,0);
++extern int vasprintf (char **, const char *, va_list) __THROWNL ATTRIBUTE_PRINTF(2,0);
+ #endif
+ 
+ /* Like vasprintf but allocates memory without fail. This works like