gcc7: port darwin changes from 5.x

pkgs/development/compilers/gcc/7/default.nix

@@ -228,6 +228,19 @@ stdenv.mkDerivation ({
 
   hardeningDisable = [ "format" ];
 
+  # This should kill all the stdinc frameworks that gcc and friends like to
+  # insert into default search paths.
+  prePatch = stdenv.lib.optionalString hostPlatform.isDarwin ''
+    substituteInPlace gcc/config/darwin-c.c \
+      --replace 'if (stdinc)' 'if (0)'
+
+    substituteInPlace libgcc/config/t-slibgcc-darwin \
+      --replace "-install_name @shlib_slibdir@/\$(SHLIB_INSTALL_NAME)" "-install_name $lib/lib/\$(SHLIB_INSTALL_NAME)"
+
+    substituteInPlace libgfortran/configure \
+      --replace "-install_name \\\$rpath/\\\$soname" "-install_name $lib/lib/\\\$soname"
+  '';
+
   postPatch =
     if (hostPlatform.isHurd
         || (libcCross != null                  # e.g., building `gcc.crossDrv'