cc/gcc: disable complibs if not selected

Force gcc to not link with some companion libraries when
there are not needed (because selected-out).

There is no option to tell gcc *not* to build the Graphite and/or
LTO stuff. They *will* be built if gcc finds the suitable companion
libraries. If we do not provide them, but the host has them, then
gcc *will* find them, and link with them.

Consider the following:
- host has suitable PPL and CLooG (eg. Debian Squeeze)
- user wants to build gcc>=4.4
- user de-selects GRAPHITE
- gcc will find the hosts PPL and CLooG, and will use them
- the user moves the toolchain to an older host that does
not have them (eg. Debian Lenny)
- the toolchain fails, when it was properly setup not to

So, explicitly tell gcc *not* to use unneeded companion libs.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

Original patch from gentoo: gentoo/src/patchsets/gcc/4.3.1/uclibc/90_all_301-missing-execinfo_h.patch

diff -durN gcc-4.3.1.orig/boehm-gc/include/gc.h gcc-4.3.1/boehm-gc/include/gc.h

--- gcc-4.3.1.orig/boehm-gc/include/gc.h	2007-04-23 23:10:09.000000000 +0200

+++ gcc-4.3.1/boehm-gc/include/gc.h	2008-06-10 14:58:54.000000000 +0200

@@ -503,7 +503,7 @@

 #if defined(__linux__) || defined(__GLIBC__)

 # include <features.h>

 # if (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 1 || __GLIBC__ > 2) \

-     && !defined(__ia64__)

+     && !defined(__ia64__) && !defined(__UCLIBC__)

 #   ifndef GC_HAVE_BUILTIN_BACKTRACE

 #     define GC_HAVE_BUILTIN_BACKTRACE

 #   endif