m4: Fix SPICE_WARNING on el6
m4: Fix SPICE_WARNING on el6

autogen.sh fails on el6 unless the second argument to
AC_DEFUN([SPICE_WARNING]) is enclosed in []

fgouget/spice-common,fgouget/spice-common,SPICE/spice-common,fgouget/spice-common,SPICE/spice-common,fgouget/spice-common,SPICE/spice-common