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>

 many ports hit this warning:

 ../sysdeps/unix/sysv/linux/check_native.c:46: warning: implicit declaration of function 'memset'

 ../sysdeps/unix/sysv/linux/check_native.c:46: warning: implicit declaration of function 'memset'

 snipped from suse

 diff -durN glibc-2.10.1.orig/sysdeps/unix/sysv/linux/check_native.c glibc-2.10.1/sysdeps/unix/sysv/linux/check_native.c

 --- glibc-2.10.1.orig/sysdeps/unix/sysv/linux/check_native.c	2007-11-24 04:12:17.000000000 +0100

 +++ glibc-2.10.1/sysdeps/unix/sysv/linux/check_native.c	2009-11-13 00:50:11.000000000 +0100

 @@ -23,6 +23,7 @@

  #include <stddef.h>

  #include <stdint.h>

  #include <stdlib.h>

 +#include <string.h>

  #include <time.h>

  #include <unistd.h>

  #include <net/if.h>