gcc5: fix for libc++ 3.8 and above

This is in preparation for the big Darwin LLVM 4.0 stdenv switchover

pkgs/development/compilers/gcc/5/default.nix

@@ -74,7 +74,10 @@ let version = "5.4.0";
       # The GNAT Makefiles did not pay attention to CFLAGS_FOR_TARGET for its
       # target libraries and tools.
       ++ optional langAda ../gnat-cflags.patch
-      ++ optional langFortran ../gfortran-driving.patch;
+      ++ optional langFortran ../gfortran-driving.patch
+
+      # This could be applied unconditionally but I don't want to cause a full Linux rebuild.
+      ++ optional stdenv.cc.isClang ./libcxx38-and-above.patch;
 
     javaEcj = fetchurl {
       # The `$(top_srcdir)/ecj.jar' file is automatically picked up at

pkgs/development/compilers/gcc/5/libcxx38-and-above.patch

@@ -0,0 +1,46 @@
+This is a slightly modified version of https://svnweb.freebsd.org/ports/head/lang/gcc5/files/patch-libc%2B%2B?revision=432958&view=co&pathrev=432958,
+which doesn't apply cleanly due to them using a slightly different format of patch from us. I just replaced the .orig file references with a/b paths.
+
+--- a/gcc/auto-profile.c 2015-01-18 02:25:42 UTC
++++ b/gcc/auto-profile.c
+@@ -19,11 +19,9 @@ along with GCC; see the file COPYING3.  
+ <http://www.gnu.org/licenses/>.  */
+ 
+ #include "config.h"
+-#include "system.h"
+-
+-#include <string.h>
+ #include <map>
+ #include <set>
++#include "system.h"
+ 
+ #include "coretypes.h"
+ #include "hash-set.h"
+--- a/gcc/graphite-isl-ast-to-gimple.c 2017-01-19 21:02:12 UTC
++++ b/gcc/graphite-isl-ast-to-gimple.c
+@@ -38,6 +38,7 @@ extern "C" {
+ #endif
+ #endif
+ 
++#include <map>
+ #include "system.h"
+ #include "coretypes.h"
+ #include "hash-set.h"
+@@ -75,7 +76,6 @@ extern "C" {
+ #include "tree-scalar-evolution.h"
+ #include "gimple-ssa.h"
+ #include "tree-into-ssa.h"
+-#include <map>
+ 
+ #ifdef HAVE_isl
+ #include "graphite-poly.h"
+--- a/gcc/system.h 2015-01-05 12:33:28 UTC
++++ b/gcc/system.h
+@@ -217,6 +217,7 @@ extern int errno;
+ #ifdef __cplusplus
+ # include <algorithm>
+ # include <cstring>
++# include <new>
+ # include <utility>
+ #endif
+