autogen.sh: tell user the correct make command

autogen.sh: tell user the correct make command

When autogen.sh finishes it helpfully prints

  "Now type 'make' to compile libvirt."

which is fine if on a host with GNU make, but on *BSD running
'make' will end in tears. We should tell users to run 'gmake'
on these platforms. If 'gmake' doesn't exist then we should
report an error too

  "GNU make is required to build libvirt"

Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>