configure: split c and cxx extra flags

There was no possibility to add specific cxx flags using the configure
file. So A new entrance has been created to support it.

Duplication of information in configure and rules.mak. Taking
QEMU_CFLAGS and add them to QEMU_CXXFLAGS, now the value of
QEMU_CXXFLAGS is stored in config-host.mak, so there is no need for
it.

The makefile for libvixl was adding flags for QEMU_CXXFLAGS in
QEMU_CFLAGS because of the addition in rules.mak. That was removed, so
adding them where it should be.

Signed-off-by: Bruno Dominguez <bru.dominguez@gmail.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1496754467-20893-1-git-send-email-bru.dominguez@gmail.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>