configure: fix check_cmd in check_host_cpp

Use correct cpp and c flags variables for the host libc.

Signed-off-by: Jörg Krause <jkrause@posteo.de>
Reviewed-by: Benoit Fouet <benoit.fouet@free.fr>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
