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>

 Correctly check for g++ existence.

 Copyright 2007 Yann E. MORIN <yann.morin.1998@anciens.enib.fr>

 Licensed to you as dmalloc-5.5.2 is.

 diff -dur dmalloc-5.5.2.orig/configure dmalloc-5.5.2/configure

 --- dmalloc-5.5.2.orig/configure	2007-05-18 11:40:31.000000000 +0200

 +++ dmalloc-5.5.2/configure	2007-05-18 11:42:02.000000000 +0200

 @@ -2643,7 +2643,7 @@

  # see if we actually have a CXX program

 -if test "$ac_cv_prog_CXX" = "" -o ! -x "$ac_cv_prog_CXX"; then

 +if test "$ac_cv_prog_CXX" = "" -o ! -x `which "$ac_cv_prog_CXX"`; then

      { echo "$as_me:$LINENO: WARNING: could not find C++ compiler $ac_cv_prog_CXX" >&5

  echo "$as_me: WARNING: could not find C++ compiler $ac_cv_prog_CXX" >&2;}

      enable_cxx=no