configure: stop allowing Mono 2.9, recommend Mono 3.2

F# 3.1 build is xbuild-based, so configure needs to check that
Mono 3.0 (or newer) is in place, not 2.9 (this old version
had limitations that prevented F# projects to build with xbuild).

We take advantage of this opportunity to re-use the AC_MSG_WARN
that recommends a mono version, to advice about using Mono 3.2
(instead of 3.0) as it defaults to SGen GC.

(Should fix #264)