libesmtp: not available for static builds

This package requires libltdl and checks for it in the configure phase.
If it's not present, then the build process is stopped.

Fixes:
   http://autobuild.buildroot.net/results/5cf/5cf145d3f9c78bef1f66fe9b1831723de8b4969f/



[Peter: dependency should only apply to COLLECTD_NOTIFY_EMAIL suboption]
Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>