autogen.sh error on no warnings

[Ruslan Kabatsayev]

Hello,

When I have no warnings from configure script, I get an error from
"cat warn" line saying "No such file or directory". I guess "cat warn"
should be changed to something like "[ -e warn ] && cat warn", or warn
should be created at the beginning of configure.
CCing Norbert Thiebaud as git-blame says the corresponding line in
configure.in was last added in commit
65335b13da74f4f350ba52493b0919a62d1a5bca.

Regards,
Ruslan