sylpheed: bump version, fix build

The old sylpheed version no longer built with our libgtk2 version, so
move to the latest stable release. At the same time add optional openssl
support.

The sylpheed build system adds -I$includedir to the CFLAGS, so ensure
it points at our staging rather than the host headers.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>