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>

 --- trunk/src/acos.c	2009/12/08 16:49:43	733

 +++ trunk/src/acos.c	2009/12/09 20:51:38	734

 @@ -188,10 +188,7 @@

    else

      rnd_im = rnd_im == GMP_RNDU ? GMP_RNDD

        : rnd_im == GMP_RNDD ? GMP_RNDU

 -#if MPFR_VERSION_MAJOR >= 3

 -      : rnd_im == GMP_RNDA ? GMP_RNDZ

 -#endif

 -      : rnd_im;

 +      : rnd_im; /* both RNDZ and RNDA map to themselves for -asin(z) */

    rnd1 = RNDC(GMP_RNDN, rnd_im);

    mpfr_init2 (pi_over_2, p);

    for (;;)