modified configure.in to fix compile bugs

There were two errors which needed to be fixed before I could compile nemo-dropbox.
1)  warning: autoconf input should be named 'configure.ac', not 'configure.in'
and
2) warning: AM_INIT_AUTOMAKE: two- and three-arguments forms are deprecated.  For more info, see:
configure.in:5: http://www.gnu.org/software/automake/manual/automake.html#Modernize-AM_005fINIT_005fAUTOMAKE-invocation

There were author warnings but they did not interfere with compilation. This pull requests fixes the two errors above.