proftpd: fix largefile file size printing

The configure script uses AC_TRY_RUN to detect if it can use the %llu format
string for printing the file size, which doesn't work when cross compiling.

We unfortunately cannot AUTORECONF the package, so instead patch configure
to force it on if we know we support it.

E.G.:

dd if=/dev/zero of=/home/ftp/file bs=1M seek=8000 count=1
1+0 records in
1+0 records out

total 1032
-rw-r--r--    1 root     root     8389656576 Jan  1 00:00 file

ncftp / > ls -l
-rw-r--r--   0        0   8389656576   Jan  1 00:00   file

VS:

ncftp / > ls -l
-rw-r--r--   0        0   4094689280   Jan  1 00:00   file

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>