configure: require glib 2.12, 2.20 for mingw32

These are pretty sane requirements to move forward with glib usage.
2.12 is the version found in RHEL/CentOS 5, and 2.20 is the
first version to support g_poll.  Without g_poll, we cannot
integrate well with the glib main loop.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
