use Debian's default root certificate location

use Debian's default root certificate location

as suggested here:
http://archives.seul.org/libevent/users/Feb-2013/msg00034.html

although curl's acinclude.m4 reveals many other possibilities:

dnl /etc/ssl/certs/ca-certificates.crt Debian systems
dnl /etc/pki/tls/certs/ca-bundle.crt Redhat and Mandriva
dnl /usr/share/ssl/certs/ca-bundle.crt old(er) Redhat
dnl /usr/local/share/certs/ca-root.crt FreeBSD
dnl /etc/ssl/cert.pem OpenBSD
dnl /etc/ssl/certs/ (ca path) SUSE

And none of these cover Windows :(