The macros under the m4 directory are outdated a bit. When trying
to compile with newer gcc I see some errors:

make[4]: Entering directory '/home/zippy/work/libvirt/libvirt-sanbox.git/libvirt-sandbox'
  CC       libvirt_sandbox_1_0_la-libvirt-sandbox-main.lo
gcc: warning: switch '-Wmudflap' is no longer supported

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>