scripts: check g++ only for host

g++ is only needed when building additonal libs on the HOST,
so check wheter g++ is available for the HOST compiler only

Signed-off-by: Bart vdr. Meulen <bartvdrmeulen@gmail.com>
[Yann E. MORIN: fix space damage]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 source "config/backend.in"

 source "config/global.in"

 source "config/target.in"

 source "config/toolchain.in"

 source "config/kernel.in"

 source "config/binutils.in"

 source "config/cc.in"

 source "config/libc.in"

 source "config/debug.in"

 source "config/companion_libs.in"

 source "config/companion_tools.in"