cc/gcc: fix C++ headers location

In case we build the C++ compiler, we have to tell gcc where to put the C++
headers, or else it will try to # put it in prefix/tuple/include, which we
make a symlink to sysroot/usr/include during the build, and that we delete
(the symlink!) after the build, but gcc will not look in sysroot/usr/inlcude
for C++ headers by default.

Implements a fix suggested by: Bryan Hundven <bryanhundven@gmail.com>

Reported-by: Anthony Foiani <anthony.foiani@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from a0924619112015bdaa81a0a2313d21ef38607bc2)

Original patch from Gentoo:

gentoo/src/patchsets/binutils/2.19/76_all_use-new-ld-dtags.patch

diff -durN binutils-2.19.orig/ld/ldmain.c binutils-2.19/ld/ldmain.c

--- binutils-2.19.orig/ld/ldmain.c	2008-11-23 16:54:42.000000000 +0100

+++ binutils-2.19/ld/ldmain.c	2008-11-23 16:56:16.000000000 +0100

@@ -262,6 +262,7 @@

   link_info.allow_undefined_version = TRUE;

   link_info.keep_memory = TRUE;

+  link_info.new_dtags = TRUE;

   link_info.combreloc = TRUE;

   link_info.strip_discarded = TRUE;

   link_info.emit_hash = TRUE;