build: move remaining compiler flag tests to meson

Remove the only remaining uses of QEMU_CFLAGS.  Now that no
feature tests are done in configure, it is possible to remove
CONFIGURE_CFLAGS and CONFIGURE_LDFLAGS as well.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>