configure: add -Werror to QEMU_CFLAGS early

configure: add -Werror to QEMU_CFLAGS early

We want all configure tests pass with -Werror if it is enabled. So we
need to update QEMU_CFLAGS early on to make sure we also pass it in to
all the compile test jobs.

This fixes a warning-became-error bug in nss for me with the default
configuration:

In file included from /usr/include/nss3/pkcs11t.h:1780,
                 from /usr/include/nss3/keythi.h:41,
                 from /usr/include/nss3/keyt.h:41,
                 from /usr/include/nss3/pk11pub.h:43,
                 from libcacard/vcard_emul_nss.c:21:
/usr/include/nss3/pkcs11n.h:365:26: error: "__GNUC_MINOR" is not defined

Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>