Stop using the $VERSION variable as it seems to be magically used by
autoconfig itself and thus gets set to the value set in AC_INIT()
without us wanting that. $LIBSSH2VER is now the libssh2 version as
detected.
Reported by: Paul Howarth
Bug: http://www.libssh2.org/mail/libssh2-devel-archive-2011-04/0008.shtml