Check for libm
Similar to commit e98f8a430f7e2fd64dab74f9b9a85ee2dfa1b92f in spice-common, channel-main.c uses sqrt(), which is found in libm on some systems. This means we are currently relying on getting -lm indirectly through some other means.
Signed-off-by: Eduardo Lima (Etrunko) etrunko@redhat.com