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>

 # powerpc specific configuration file

 config ARCH_POWERPC_SPE

     bool

     prompt "Enable SPE support"

     default n

     help

       Add support for the Signal Processing Engine.  This will set up

       the toolchain so that it supports the SPE ABI extensions. This

       mainly targets Freescale e500 processors.

       Setting this option will append "spe" to the end of your target

       tuple name (e.g., powerpc-e500v2-linux-gnuspe) so that the gcc

       configure/build system will know to include SPE ABI support.

       It will also automatically add "-mabi=spe -mspe" to your

       TARGET_CFLAGS, and "--enable-e500_double" to your CC_EXTRA_CONFIG,

       so you do not need to explicitly add them.